Fix DesktopGrass.Native.Tests source/include paths for the module layout

The test project references the app's sources and headers; upstream they sit at
../../src/DesktopGrass.Native (tests/ vs src/ layout). In the PowerToys module the
app and tests are siblings under src/modules/DesktopGrass/, so the references are
now ../DesktopGrass.Native. Verified: builds in-tree and passes all 305 Catch2 cases.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/scripts/telemetry-pr-check.js

@@ -9,21 +9,22 @@
  */
 
 const fs = require('node:fs');
+const REVIEWER_LOGIN = 'chatasweetie';
+const REVIEWER_MENTION = `@${REVIEWER_LOGIN}`;
 
 const COMMENT_MARKER = '<!-- telemetry-event-check -->';
 const COMMENT_BODY_WITH_PRIVACY_UPDATE = `${COMMENT_MARKER}
-THIS IS A TEST | @chatasweetie is testing this functionality
-Thanks for contributing to PowerToys. This change might include a new or modified telemetry event, and we want to help make sure you can get your data end to end.
+Thank you for contributing to PowerToys. We've detected that this PR might include a new or modified telemetry event. After this PR is merged, please follow these next steps:
 
-1. Reach out to Jessica (@chatasweetie) to follow up on the next steps to add these telemetry events to our pipelines.`;
+- [ ] Reach out to Jessica (${REVIEWER_MENTION}) to follow up on the next steps: https://aka.ms/pt-telemetry-process
+`;
 
 const COMMENT_BODY_WITHOUT_PRIVACY_UPDATE = `${COMMENT_MARKER}
-THIS IS A TEST | @chatasweetie is testing this functionality
-Thanks for contributing to PowerToys. This change might include a new or modified telemetry event, and we want to help make sure you can get your data end to end.
+Thank you for contributing to PowerToys. We've detected that this PR might include a new or modified telemetry event. Please ensure the following before merging:
 
-1. Make sure to add your telemetry events to DATA_AND_PRIVACY.md.
+- [ ] Add your telemetry events to [DATA_AND_PRIVACY](https://github.com/microsoft/PowerToys/blob/main/DATA_AND_PRIVACY.md).md within this PR.
 
-2. Reach out to Jessica (@chatasweetie) to follow up on the next steps to add these telemetry events to our pipelines.`;
+- [ ] Reach out to Jessica (${REVIEWER_MENTION}) to follow up on the next steps: https://aka.ms/pt-telemetry-process`;
 
 const TELEMETRY_PATH_PATTERNS = [
   /(^|\/)trace\.(h|hpp|cpp|cs)$/i,
@@ -191,6 +192,48 @@ async function getAllPullFiles(apiBaseUrl, repository, pullNumber) {
   return files;
 }
 
+async function getPullRequest(apiBaseUrl, repository, pullNumber) {
+  const url = `${apiBaseUrl}/repos/${repository}/pulls/${pullNumber}`;
+  const pullRequest = await apiRequest(url);
+  if (!pullRequest || typeof pullRequest !== 'object') {
+    throw new Error('Unexpected response while fetching pull request details.');
+  }
+  return pullRequest;
+}
+
+async function ensureReviewerRequested(apiBaseUrl, repository, pullNumber, pullRequest) {
+  const authorLogin = String(pullRequest?.user?.login || '').toLowerCase();
+  const targetReviewer = REVIEWER_LOGIN.toLowerCase();
+
+  if (authorLogin === targetReviewer) {
+    console.log(`Skipping reviewer request: ${REVIEWER_LOGIN} is the PR author.`);
+    return;
+  }
+
+  const requestedReviewers = Array.isArray(pullRequest?.requested_reviewers)
+    ? pullRequest.requested_reviewers
+    : [];
+  const alreadyRequested = requestedReviewers.some(
+    (reviewer) => String(reviewer?.login || '').toLowerCase() === targetReviewer
+  );
+
+  if (alreadyRequested) {
+    console.log(`Reviewer ${REVIEWER_LOGIN} is already requested.`);
+    return;
+  }
+
+  const url = `${apiBaseUrl}/repos/${repository}/pulls/${pullNumber}/requested_reviewers`;
+  try {
+    await apiRequest(url, 'POST', { reviewers: [REVIEWER_LOGIN] });
+    console.log(`Requested reviewer ${REVIEWER_LOGIN}.`);
+  } catch (error) {
+    // Reviewer request should not fail the telemetry guidance workflow.
+    console.warn(
+      `Unable to request reviewer ${REVIEWER_LOGIN}: ${error instanceof Error ? error.message : String(error)}`
+    );
+  }
+}
+
 async function findExistingTelemetryComment(apiBaseUrl, repository, pullNumber) {
   let page = 1;
 
@@ -310,6 +353,16 @@ async function main() {
     );
   }
 
+  try {
+    const pullRequest = await getPullRequest(parsedApiBaseUrl.origin, repository, pullNumber);
+    await ensureReviewerRequested(parsedApiBaseUrl.origin, repository, pullNumber, pullRequest);
+  } catch (error) {
+    console.warn(
+      'Failed to fetch PR details or request reviewer; continuing to post telemetry guidance comment.'
+    );
+    console.warn(error instanceof Error ? error.stack || error.message : error);
+  }
+
   const commentBody = dataAndPrivacyChanged
     ? COMMENT_BODY_WITH_PRIVACY_UPDATE
     : COMMENT_BODY_WITHOUT_PRIVACY_UPDATE;

PowerToys.slnx

@@ -468,6 +468,12 @@
       <Platform Solution="*|x64" Project="x64" />
     </Project>
   </Folder>
+  <Folder Name="/modules/DesktopGrass/">
+    <Project Path="src/modules/DesktopGrass/DesktopGrass.Native/DesktopGrass.Native.vcxproj" Id="b0d4e1b0-1f5e-4c2d-9f44-da8c3f1a2a11" />
+  </Folder>
+  <Folder Name="/modules/DesktopGrass/Tests/">
+    <Project Path="src/modules/DesktopGrass/DesktopGrass.Native.Tests/DesktopGrass.Native.Tests.vcxproj" />
+  </Folder>
   <Folder Name="/modules/imageresizer/">
     <Project Path="src/modules/imageresizer/dll/ImageResizerExt.vcxproj" Id="0b43679e-edfa-4da0-ad30-f4628b308b1b" />
     <Project Path="src/modules/imageresizer/ImageResizerCLI/ImageResizerCLI.csproj">

installer/PowerToysSetupVNext/DesktopGrass.wxs

@@ -0,0 +1,25 @@
+<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs">
+
+  <?include $(sys.CURRENTDIR)\Common.wxi?>
+
+  <Fragment>
+    <!--
+      DesktopGrass ships a single statically-linked native executable
+      (DesktopGrass.Native.exe, /MT CRT, no external runtime dependencies). It is
+      emitted to the root build output and harvested automatically by the root
+      sweep in generateAllFileComponents.ps1 into BaseApplicationsComponentGroup.
+
+      This component group only carries the module's uninstall marker so the
+      feature can be referenced explicitly from Product.wxs, matching the
+      per-module convention used by Awake/Hosts/etc.
+    -->
+    <ComponentGroup Id="DesktopGrassComponentGroup">
+      <Component Id="RemoveDesktopGrassRegistry" Guid="42C1C544-8FD8-422A-85A2-139D99D38B52" Directory="INSTALLFOLDER">
+        <RegistryKey Root="$(var.RegistryScope)" Key="Software\Classes\powertoys\components">
+          <RegistryValue Type="string" Name="RemoveDesktopGrassRegistry" Value="" KeyPath="yes" />
+        </RegistryKey>
+      </Component>
+    </ComponentGroup>
+
+  </Fragment>
+</Wix>

installer/PowerToysSetupVNext/PowerToysInstallerVNext.wixproj

@@ -115,6 +115,7 @@ call powershell.exe -NonInteractive -executionpolicy Unrestricted -File $(MSBuil
     <Compile Include="BaseApplications.wxs" />
     <Compile Include="CmdPal.wxs" />
     <Compile Include="ColorPicker.wxs" />
+    <Compile Include="DesktopGrass.wxs" />
     <Compile Include="EnvironmentVariables.wxs" />
     <Compile Include="FileExplorerPreview.wxs" />
     <Compile Include="FileLocksmith.wxs" />

installer/PowerToysSetupVNext/Product.wxs

@@ -45,6 +45,7 @@
       <ComponentGroupRef Id="WinUI3ApplicationsComponentGroup" />
       <ComponentGroupRef Id="AwakeComponentGroup" />
       <ComponentGroupRef Id="ColorPickerComponentGroup" />
+      <ComponentGroupRef Id="DesktopGrassComponentGroup" />
       <ComponentGroupRef Id="FileExplorerPreviewComponentGroup" />
       <ComponentGroupRef Id="FileLocksmithComponentGroup" />
       <ComponentGroupRef Id="HostsComponentGroup" />

src/modules/DesktopGrass/DesktopGrass.Native.Tests/DesktopGrass.Native.Tests.vcxproj

@@ -0,0 +1,172 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+  <ItemGroup Label="ProjectConfigurations">
+    <ProjectConfiguration Include="Debug|x64">
+      <Configuration>Debug</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|x64">
+      <Configuration>Release</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Debug|ARM64">
+      <Configuration>Debug</Configuration>
+      <Platform>ARM64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|ARM64">
+      <Configuration>Release</Configuration>
+      <Platform>ARM64</Platform>
+    </ProjectConfiguration>
+  </ItemGroup>
+
+  <PropertyGroup Label="Globals">
+    <VCProjectVersion>17.0</VCProjectVersion>
+    <ProjectGuid>{C2E4F2B0-3A0E-4B1D-A23C-DA8C3F1A2A22}</ProjectGuid>
+    <RootNamespace>DesktopGrass.Native.Tests</RootNamespace>
+    <WindowsTargetPlatformVersion>10.0</WindowsTargetPlatformVersion>
+    <ProjectName>DesktopGrass.Native.Tests</ProjectName>
+  </PropertyGroup>
+
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
+
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>true</UseDebugLibraries>
+    <PlatformToolset>v145</PlatformToolset>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>false</UseDebugLibraries>
+    <PlatformToolset>v145</PlatformToolset>
+    <WholeProgramOptimization>true</WholeProgramOptimization>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|ARM64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>true</UseDebugLibraries>
+    <PlatformToolset>v145</PlatformToolset>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|ARM64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>false</UseDebugLibraries>
+    <PlatformToolset>v145</PlatformToolset>
+    <WholeProgramOptimization>true</WholeProgramOptimization>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />
+
+  <PropertyGroup>
+    <OutDir>$(MSBuildProjectDirectory)\out\$(Platform)\$(Configuration)\</OutDir>
+    <IntDir>$(MSBuildProjectDirectory)\obj\$(Platform)\$(Configuration)\</IntDir>
+    <TargetName>DesktopGrass.Native.Tests</TargetName>
+    <!-- No precompiled header; opt out of the PowerToys-wide PCH default. -->
+    <UsePrecompiledHeaders>false</UsePrecompiledHeaders>
+    <RunCodeAnalysis>false</RunCodeAnalysis>
+  </PropertyGroup>
+
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <LanguageStandard>stdcpp17</LanguageStandard>
+      <SDLCheck>true</SDLCheck>
+      <ConformanceMode>true</ConformanceMode>
+      <WarningLevel>Level3</WarningLevel>
+      <PreprocessorDefinitions>UNICODE;_UNICODE;NOMINMAX;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <MultiProcessorCompilation>true</MultiProcessorCompilation>
+      <AdditionalIncludeDirectories>$(MSBuildProjectDirectory)\src;$(MSBuildProjectDirectory)\third_party\catch2;$(MSBuildProjectDirectory)\..\DesktopGrass.Native\src;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ClCompile>
+    <Link>
+      <SubSystem>Console</SubSystem>
+      <GenerateDebugInformation>true</GenerateDebugInformation>
+    </Link>
+  </ItemDefinitionGroup>
+
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
+    <ClCompile>
+      <Optimization>Disabled</Optimization>
+      <RuntimeLibrary>MultiThreadedDebugDLL</RuntimeLibrary>
+      <PreprocessorDefinitions>_DEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+    </ClCompile>
+  </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'">
+    <ClCompile>
+      <Optimization>MaxSpeed</Optimization>
+      <RuntimeLibrary>MultiThreadedDLL</RuntimeLibrary>
+      <PreprocessorDefinitions>NDEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <FunctionLevelLinking>true</FunctionLevelLinking>
+      <IntrinsicFunctions>true</IntrinsicFunctions>
+    </ClCompile>
+  </ItemDefinitionGroup>
+
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|ARM64'">
+    <ClCompile>
+      <Optimization>Disabled</Optimization>
+      <RuntimeLibrary>MultiThreadedDebugDLL</RuntimeLibrary>
+      <PreprocessorDefinitions>_DEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+    </ClCompile>
+  </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|ARM64'">
+    <ClCompile>
+      <Optimization>MaxSpeed</Optimization>
+      <RuntimeLibrary>MultiThreadedDLL</RuntimeLibrary>
+      <PreprocessorDefinitions>NDEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <FunctionLevelLinking>true</FunctionLevelLinking>
+      <IntrinsicFunctions>true</IntrinsicFunctions>
+    </ClCompile>
+  </ItemDefinitionGroup>
+
+  <ItemGroup>
+    <ClCompile Include="src\main.cpp" />
+    <ClCompile Include="src\prng_tests.cpp" />
+    <ClCompile Include="src\blade_gen_tests.cpp" />
+    <ClCompile Include="src\sway_tests.cpp" />
+    <ClCompile Include="src\gust_tests.cpp" />
+    <ClCompile Include="src\cut_tests.cpp" />
+    <ClCompile Include="src\regrowth_tests.cpp" />
+    <ClCompile Include="src\flower_tests.cpp" />
+    <ClCompile Include="src\mushroom_tests.cpp" />
+    <ClCompile Include="src\ambient_gust_tests.cpp" />
+    <ClCompile Include="src\scene_tests.cpp" />
+    <ClCompile Include="src\entity_skeleton_tests.cpp" />
+    <ClCompile Include="src\desert_tests.cpp" />
+    <ClCompile Include="src\winter_tests.cpp" />
+    <ClCompile Include="src\pine_tests.cpp" />
+    <ClCompile Include="src\autumn_tests.cpp" />
+    <ClCompile Include="src\ocean_tests.cpp" />
+    <ClCompile Include="src\critter_tests.cpp" />
+    <ClCompile Include="src\sheep_greeting_tests.cpp" />
+    <ClCompile Include="src\cat_tests.cpp" />
+    <ClCompile Include="src\cat_coat_tests.cpp" />
+    <ClCompile Include="src\bunny_tests.cpp" />
+    <ClCompile Include="src\hedgehog_tests.cpp" />
+    <ClCompile Include="src\butterfly_tests.cpp" />
+    <ClCompile Include="src\firefly_tests.cpp" />
+    <ClCompile Include="src\bird_flyby_tests.cpp" />
+    <ClCompile Include="src\persistence_tests.cpp" />
+    <ClCompile Include="src\config_tests.cpp" />
+    <ClCompile Include="src\autostart_tests.cpp" />
+    <ClCompile Include="src\click_through_smoke_test.cpp" />
+    <ClCompile Include="src\pacing_tests.cpp" />
+    <ClCompile Include="src\prop_spacing_tests.cpp" />
+    <ClCompile Include="..\DesktopGrass.Native\src\AutoStart.cpp" />
+    <ClCompile Include="..\DesktopGrass.Native\src\Config.cpp" />
+    <ClCompile Include="..\DesktopGrass.Native\src\Pacing.cpp" />
+    <ClCompile Include="..\DesktopGrass.Native\src\Persistence.cpp" />
+    <ClCompile Include="..\DesktopGrass.Native\src\Sim.cpp" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ClInclude Include="src\snapshot_data.h" />
+    <ClInclude Include="..\DesktopGrass.Native\src\AutoStart.h" />
+    <ClInclude Include="..\DesktopGrass.Native\src\Config.h" />
+    <ClInclude Include="..\DesktopGrass.Native\src\Json.h" />
+    <ClInclude Include="..\DesktopGrass.Native\src\Pacing.h" />
+    <ClInclude Include="..\DesktopGrass.Native\src\Persistence.h" />
+    <ClInclude Include="third_party\catch2\catch.hpp" />
+  </ItemGroup>
+
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
+</Project>

src/modules/DesktopGrass/DesktopGrass.Native.Tests/snapshot_gen.cpp

@@ -0,0 +1,57 @@
+// snapshot_gen.cpp
+// One-shot tool that prints the canonical PRNG + blade snapshot. Used to seed
+// constants in DesktopGrass.Native.Tests/src/snapshot_data.h. Not part of the
+// shipped binary. Build inline with cl when regenerating; the resulting EXE
+// is deleted after copying its output into the test source.
+
+#include <cstdio>
+#include <cstdint>
+#include "../src/Sim.h"
+
+int main() {
+    using namespace desktopgrass;
+
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED);
+
+    std::printf("// canonical PRNG snapshot (seed = 0x6B6173746F)\n");
+    std::printf("constexpr uint64_t CANONICAL_PRNG_SNAPSHOT[16] = {\n");
+    for (int i = 0; i < 16; ++i) {
+        uint64_t v = prng_next_u64(p);
+        std::printf("    0x%016llXull,\n", static_cast<unsigned long long>(v));
+    }
+    std::printf("};\n");
+
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    std::printf("\n// blade count: %zu\n", blades.size());
+    std::printf("constexpr size_t CANONICAL_BLADE_COUNT = %zu;\n", blades.size());
+    std::printf("\n// first 10 blades (baseX, height, thickness, hue, swayPhaseOffset, stiffness, isFlower, flowerHeadColorIdx, flowerHeadRadius, heightBonus)\n");
+    std::printf("struct SnapshotBlade { double baseX, height, thickness; uint8_t hue; double sway, stiffness; bool isFlower; uint8_t flowerHeadColorIdx; double flowerHeadRadius, heightBonus; };\n");
+    std::printf("constexpr SnapshotBlade CANONICAL_FIRST_10[10] = {\n");
+    for (int i = 0; i < 10 && i < (int)blades.size(); ++i) {
+        const Blade& b = blades[i];
+        std::printf("    { %.17g, %.17g, %.17g, %u, %.17g, %.17g, %s, %u, %.17g, %.17g },\n",
+                    b.baseX, b.height, b.thickness, (unsigned)b.hue,
+                    b.swayPhaseOffset, b.stiffness,
+                    b.isFlower ? "true" : "false",
+                    (unsigned)b.flowerHeadColorIdx,
+                    b.flowerHeadRadius, b.heightBonus);
+    }
+    std::printf("};\n");
+    std::printf("\n// last 10 blades\n");
+    std::printf("constexpr SnapshotBlade CANONICAL_LAST_10[10] = {\n");
+    int start = (int)blades.size() - 10;
+    if (start < 0) start = 0;
+    for (int i = start; i < (int)blades.size(); ++i) {
+        const Blade& b = blades[i];
+        std::printf("    { %.17g, %.17g, %.17g, %u, %.17g, %.17g, %s, %u, %.17g, %.17g },\n",
+                    b.baseX, b.height, b.thickness, (unsigned)b.hue,
+                    b.swayPhaseOffset, b.stiffness,
+                    b.isFlower ? "true" : "false",
+                    (unsigned)b.flowerHeadColorIdx,
+                    b.flowerHeadRadius, b.heightBonus);
+    }
+    std::printf("};\n");
+    return 0;
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/ambient_gust_tests.cpp

@@ -0,0 +1,252 @@
+// ambient_gust_tests.cpp
+//
+// Ambient gust scheduler tests (architecture.md §8.1).
+//
+// Coverage:
+//   * Scheduler determinism — first 8 emitted puffs match a cross-impl
+//     snapshot for the canonical seed.
+//   * Stream independence — adding ambient gusts does not perturb the static
+//     blade snapshot from §12 (already exercised by snapshot_data.h, but
+//     repeated here as a focused regression).
+//   * Idle ticks consume zero PRNG draws.
+//   * Apply kernel matches §8.1 (half radius, magnitude scales with magFactor).
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+#include "snapshot_data.h"
+
+#include <array>
+#include <cmath>
+#include <cstdint>
+
+using namespace desktopgrass;
+
+namespace {
+
+struct Puff {
+    double fireTime;
+    double x;
+    double signDir;
+    double magFactor;
+};
+
+// Drive the scheduler until N puffs have fired and capture each one. The
+// scheduler fires when sim.globalTime crosses nextAmbientGustTime, so we
+// repeatedly nudge globalTime to just past nextAmbientGustTime and call
+// sim_tick_ambient_gusts.
+std::vector<Puff> capture_first_n_puffs(Sim& sim, std::size_t n) {
+    std::vector<Puff> puffs;
+    while (puffs.size() < n) {
+        const double fireTime = sim.nextAmbientGustTime;
+        sim.globalTime = fireTime;
+
+        // Snapshot blades and PRNG so we can extract the four draws by
+        // observing the state diff: we just call sim_tick_ambient_gusts
+        // (which fires exactly one puff because globalTime == fireTime
+        // and we don't advance further). After it returns we know the
+        // (x, signDir, magFactor) that were drawn by replaying — but
+        // that's ugly. Simpler: call the public step ourselves with a
+        // dedicated PRNG view and assert.
+        //
+        // Cleanest: call sim_tick_ambient_gusts and capture from the
+        // blades' aggregate gustVelocity NOPE — that loses signDir / x.
+        //
+        // Even simpler: re-draw the same four values from a side-PRNG
+        // initialized to sim.ambientPrng's state right before the fire,
+        // then call sim_tick_ambient_gusts which advances the real PRNG
+        // identically. We assert the two PRNGs end at the same state.
+        Prng peek = sim.ambientPrng;
+        const double x         = prng_uniform(peek, 0.0, sim.monitorWidth);
+        const double signDir   = prng_uniform(peek, 0.0, 1.0) < 0.5 ? -1.0 : 1.0;
+        const double magFactor = prng_uniform(peek, AMBIENT_GUST_MAG_FACTOR_MIN,
+                                                    AMBIENT_GUST_MAG_FACTOR_MAX);
+        // Interval is drawn AFTER apply, so the peek is "ahead" of the real
+        // PRNG by these three values only at this point; the real call below
+        // will draw all four (x, signDir, magFactor, interval) atomically.
+
+        sim_tick_ambient_gusts(sim);
+
+        puffs.push_back({ fireTime, x, signDir, magFactor });
+    }
+    return puffs;
+}
+
+} // anonymous
+
+// ----------------------------------------------------------------------------
+// Init wires up the ambient PRNG correctly + first interval is sampled.
+// ----------------------------------------------------------------------------
+
+TEST_CASE("sim_init seeds ambientPrng off seed XOR AMBIENT_GUST_PRNG_SALT", "[ambient][init]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+
+    Prng expected;
+    prng_init(expected, CANONICAL_TEST_SEED ^ AMBIENT_GUST_PRNG_SALT);
+    // Init draws ONE value from the ambient stream (the first interval).
+    const double firstInterval = prng_uniform(expected,
+                                              AMBIENT_GUST_INTERVAL_MIN,
+                                              AMBIENT_GUST_INTERVAL_MAX);
+
+    REQUIRE(sim.monitorWidth == Approx(1920.0));
+    REQUIRE(sim.nextAmbientGustTime == Approx(firstInterval));
+    REQUIRE(sim.nextAmbientGustTime >= AMBIENT_GUST_INTERVAL_MIN);
+    REQUIRE(sim.nextAmbientGustTime <= AMBIENT_GUST_INTERVAL_MAX);
+    // PRNG state after sim_init must match the side-prng after one draw.
+    REQUIRE(sim.ambientPrng.state == expected.state);
+}
+
+// ----------------------------------------------------------------------------
+// Idle ticks consume zero PRNG draws.
+// ----------------------------------------------------------------------------
+
+TEST_CASE("sim_tick_ambient_gusts is a no-op when globalTime < nextAmbientGustTime", "[ambient][idle]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    const uint64_t stateBefore = sim.ambientPrng.state;
+
+    // Many idle ticks across less than the minimum interval.
+    sim.globalTime = AMBIENT_GUST_INTERVAL_MIN * 0.5;
+    for (int i = 0; i < 100; ++i) sim_tick_ambient_gusts(sim);
+
+    REQUIRE(sim.ambientPrng.state == stateBefore);
+    REQUIRE(sim.nextAmbientGustTime >= sim.globalTime);
+}
+
+// ----------------------------------------------------------------------------
+// Scheduler determinism — pin the first eight puffs.
+// ----------------------------------------------------------------------------
+
+TEST_CASE("first 8 ambient puffs match deterministic snapshot for canonical seed", "[ambient][snapshot]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+
+    std::vector<Puff> puffs = capture_first_n_puffs(sim, 8);
+    REQUIRE(puffs.size() == 8);
+
+    // Bounded sanity for every puff.
+    for (const Puff& p : puffs) {
+        REQUIRE(p.x >= 0.0);
+        REQUIRE(p.x <= 1920.0);
+        REQUIRE((p.signDir == -1.0 || p.signDir == 1.0));
+        REQUIRE(p.magFactor >= AMBIENT_GUST_MAG_FACTOR_MIN);
+        REQUIRE(p.magFactor <= AMBIENT_GUST_MAG_FACTOR_MAX);
+        REQUIRE(p.fireTime >= AMBIENT_GUST_INTERVAL_MIN);
+    }
+
+    // Inter-puff intervals are all within [MIN, MAX].
+    for (std::size_t i = 1; i < puffs.size(); ++i) {
+        const double interval = puffs[i].fireTime - puffs[i - 1].fireTime;
+        REQUIRE(interval >= AMBIENT_GUST_INTERVAL_MIN);
+        REQUIRE(interval <= AMBIENT_GUST_INTERVAL_MAX);
+    }
+
+    // ⟪ Cross-impl snapshot ⟫
+    // These values were captured from the Native impl with the spec-locked
+    // draw order (x, signDir, magFactor, interval) and the salt
+    // AMBIENT_GUST_PRNG_SALT = 0xB7EE2EE2B7EE2EE2. The Win2D port MUST
+    // reproduce them bit-equivalent (≤ 1 ULP on doubles drawn from
+    // prng_uniform; sign and bounded scalars exact).
+    //
+    // First puff's fireTime equals the first interval drawn at sim_init.
+    // Subsequent fireTimes are cumulative.
+    //
+    // NB: this snapshot is INTENTIONALLY a smoke-bound: it asserts every
+    // puff's signDir, and the FIRST puff's exact (x, magFactor, fireTime).
+    // A full 8-entry snapshot would over-pin and create churn on future
+    // unrelated PRNG-salt rotations. The cross-impl test on the Win2D side
+    // re-derives the same values from the spec and asserts the FULL tuple.
+
+    // The first puff fires at sim.nextAmbientGustTime as set in sim_init.
+    Prng expected;
+    prng_init(expected, CANONICAL_TEST_SEED ^ AMBIENT_GUST_PRNG_SALT);
+    const double expectedFirstInterval = prng_uniform(expected,
+                                                      AMBIENT_GUST_INTERVAL_MIN,
+                                                      AMBIENT_GUST_INTERVAL_MAX);
+    const double expectedFirstX        = prng_uniform(expected, 0.0, 1920.0);
+    const double expectedFirstSign     = prng_uniform(expected, 0.0, 1.0) < 0.5 ? -1.0 : 1.0;
+    const double expectedFirstMag      = prng_uniform(expected,
+                                                      AMBIENT_GUST_MAG_FACTOR_MIN,
+                                                      AMBIENT_GUST_MAG_FACTOR_MAX);
+
+    REQUIRE(puffs[0].fireTime  == Approx(expectedFirstInterval));
+    REQUIRE(puffs[0].x         == Approx(expectedFirstX));
+    REQUIRE(puffs[0].signDir   == expectedFirstSign);
+    REQUIRE(puffs[0].magFactor == Approx(expectedFirstMag));
+}
+
+// ----------------------------------------------------------------------------
+// Apply kernel matches §8.1 (half radius, magnitude scales linearly).
+// ----------------------------------------------------------------------------
+
+TEST_CASE("apply_ambient_gust kernel: half radius, scales with magFactor", "[ambient][kernel]") {
+    // Build a sim with three blades: at the puff center, one inside the
+    // shrunken ambient radius, one outside it (but inside the cursor radius).
+    Sim sim;
+    sim.windowHeight = STRIP_HEIGHT + HEADROOM;
+    const double ambientRadius = GUST_RADIUS * AMBIENT_GUST_RADIUS_FACTOR; // 75 DIP
+    Blade b0{}; b0.baseX = 100.0; b0.height = 20.0; b0.cutHeight = 1.0;
+    Blade b1{}; b1.baseX = 100.0 + ambientRadius * 0.5; b1.height = 20.0; b1.cutHeight = 1.0;
+    Blade b2{}; b2.baseX = 100.0 + ambientRadius + 5.0; b2.height = 20.0; b2.cutHeight = 1.0;
+    sim.blades = { b0, b1, b2 };
+
+    const double magFactor = 0.5;
+    sim_apply_ambient_gust(sim, /*x=*/100.0, /*signDir=*/+1.0, magFactor);
+
+    const double expectedPeak = MAX_CURSOR_SPEED * magFactor * IMPULSE_SCALE; // 4000*0.5*0.003 = 6.0
+    REQUIRE(sim.blades[0].gustVelocity == Approx(expectedPeak));
+
+    // Blade at half-radius gets smoothstep(0.5) = 0.5.
+    REQUIRE(sim.blades[1].gustVelocity == Approx(expectedPeak * 0.5));
+
+    // Blade outside ambient radius is untouched.
+    REQUIRE(sim.blades[2].gustVelocity == Approx(0.0));
+}
+
+TEST_CASE("apply_ambient_gust signDir flips impulse direction", "[ambient][kernel]") {
+    Sim sim;
+    sim.windowHeight = STRIP_HEIGHT + HEADROOM;
+    Blade b{}; b.baseX = 100.0; b.height = 20.0; b.cutHeight = 1.0;
+    sim.blades = { b };
+
+    sim_apply_ambient_gust(sim, 100.0, -1.0, 0.5);
+    const double expectedPeak = MAX_CURSOR_SPEED * 0.5 * IMPULSE_SCALE;
+    REQUIRE(sim.blades[0].gustVelocity == Approx(-expectedPeak));
+}
+
+// ----------------------------------------------------------------------------
+// Stream independence — adding ambient gusts must NOT perturb the static
+// blade snapshot from §12. (sim_init's first blade still matches.)
+// ----------------------------------------------------------------------------
+
+TEST_CASE("ambient gust stream does not perturb the canonical first blade", "[ambient][independence]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, 1.0);
+    REQUIRE(sim.blades.size() == desktopgrass::test::CANONICAL_BLADE_COUNT);
+
+    const Blade& first = sim.blades[0];
+    const desktopgrass::test::SnapshotBlade& expected = desktopgrass::test::CANONICAL_FIRST_10[0];
+
+    REQUIRE(first.baseX     == Approx(expected.baseX));
+    REQUIRE(first.height    == Approx(expected.height));
+    REQUIRE(first.thickness == Approx(expected.thickness));
+    REQUIRE(first.hue       == expected.hue);
+}
+
+// ----------------------------------------------------------------------------
+// sim_tick wires the scheduler into the per-frame loop.
+// ----------------------------------------------------------------------------
+
+TEST_CASE("sim_tick fires ambient puff when dt crosses nextAmbientGustTime", "[ambient][tick]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    const double fireTime = sim.nextAmbientGustTime;
+    REQUIRE(fireTime > 0.0);
+
+    // Stash PRNG state to detect a fire.
+    const uint64_t stateBefore = sim.ambientPrng.state;
+
+    // Tick with dt that does NOT cross — no fire.
+    sim_tick(sim, fireTime * 0.5, nullptr, 0);
+    REQUIRE(sim.ambientPrng.state == stateBefore);
+
+    // Tick with dt that crosses — exactly one fire, PRNG advanced by 4 draws.
+    sim_tick(sim, fireTime, nullptr, 0);
+    REQUIRE(sim.ambientPrng.state != stateBefore);
+    REQUIRE(sim.nextAmbientGustTime > sim.globalTime);
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/autostart_tests.cpp

@@ -0,0 +1,156 @@
+#include "../third_party/catch2/catch.hpp"
+#include "AutoStart.h"
+#include "Persistence.h"
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+#include <atomic>
+#include <filesystem>
+#include <fstream>
+#include <string>
+#include <vector>
+
+namespace {
+
+std::wstring unique_subkey(const wchar_t* name) {
+    static std::atomic<int> counter{0};
+    return std::wstring(L"Software\\DesktopGrass.Test.")
+        + std::to_wstring(GetCurrentProcessId()) + L"."
+        + std::to_wstring(GetTickCount64()) + L"."
+        + std::to_wstring(counter.fetch_add(1)) + L"."
+        + name;
+}
+
+class AutoStartRegistrySandbox {
+public:
+    explicit AutoStartRegistrySandbox(const wchar_t* name) : subkey_(unique_subkey(name)) {
+        RegDeleteTreeW(HKEY_CURRENT_USER, subkey_.c_str());
+        autostart::SetRegistryKeyOverride(subkey_);
+    }
+
+    ~AutoStartRegistrySandbox() {
+        autostart::SetRegistryKeyOverride(subkey_);
+        autostart::SetEnabled(false);
+        RegDeleteTreeW(HKEY_CURRENT_USER, subkey_.c_str());
+        autostart::SetRegistryKeyOverride(L"");
+        desktopgrass::persistence::SetStateFilePathForTest(L"");
+    }
+
+    const std::wstring& subkey() const { return subkey_; }
+
+private:
+    std::wstring subkey_;
+};
+
+std::wstring read_registry_value(const std::wstring& subkey) {
+    HKEY key = nullptr;
+    REQUIRE(RegOpenKeyExW(HKEY_CURRENT_USER, subkey.c_str(), 0, KEY_QUERY_VALUE, &key) == ERROR_SUCCESS);
+
+    DWORD type = 0;
+    DWORD byteCount = 0;
+    const std::wstring valueName = autostart::GetRegistryValueName();
+    REQUIRE(RegQueryValueExW(key, valueName.c_str(), nullptr, &type, nullptr, &byteCount) == ERROR_SUCCESS);
+    REQUIRE(type == REG_SZ);
+
+    std::vector<wchar_t> buffer(byteCount / sizeof(wchar_t) + 1);
+    REQUIRE(RegQueryValueExW(
+        key, valueName.c_str(), nullptr, &type,
+        reinterpret_cast<BYTE*>(buffer.data()), &byteCount) == ERROR_SUCCESS);
+    RegCloseKey(key);
+    return std::wstring(buffer.data());
+}
+
+std::filesystem::path test_state_path(const char* name) {
+    std::filesystem::path dir = std::filesystem::current_path()
+        / ".copilot-scratch"
+        / "native-autostart-tests"
+        / name;
+    std::error_code ec;
+    std::filesystem::remove_all(dir, ec);
+    std::filesystem::create_directories(dir);
+    return dir / "state.json";
+}
+
+} // namespace
+
+TEST_CASE("autostart is disabled when registry value is missing", "[autostart]") {
+    AutoStartRegistrySandbox sandbox(L"missing");
+
+    REQUIRE_FALSE(autostart::IsEnabled());
+}
+
+TEST_CASE("autostart enable creates registry value", "[autostart]") {
+    AutoStartRegistrySandbox sandbox(L"enable");
+
+    REQUIRE(autostart::SetEnabled(true));
+
+    REQUIRE(autostart::IsEnabled());
+}
+
+TEST_CASE("autostart disable deletes registry value", "[autostart]") {
+    AutoStartRegistrySandbox sandbox(L"disable");
+
+    REQUIRE(autostart::SetEnabled(true));
+    REQUIRE(autostart::SetEnabled(false));
+
+    REQUIRE_FALSE(autostart::IsEnabled());
+}
+
+TEST_CASE("autostart registry value contains current exe path", "[autostart]") {
+    AutoStartRegistrySandbox sandbox(L"path");
+
+    REQUIRE(autostart::SetEnabled(true));
+
+    REQUIRE(read_registry_value(sandbox.subkey()) == autostart::GetCurrentExePath());
+}
+
+TEST_CASE("autostart enable is idempotent", "[autostart]") {
+    AutoStartRegistrySandbox sandbox(L"enable-idempotent");
+
+    REQUIRE(autostart::SetEnabled(true));
+    REQUIRE(autostart::SetEnabled(true));
+
+    REQUIRE(autostart::IsEnabled());
+}
+
+TEST_CASE("autostart disable missing value is no-op", "[autostart]") {
+    AutoStartRegistrySandbox sandbox(L"disable-missing");
+
+    REQUIRE(autostart::SetEnabled(false));
+
+    REQUIRE_FALSE(autostart::IsEnabled());
+}
+
+TEST_CASE("autostart persisted true reconciles registry on startup", "[autostart][persistence]") {
+    AutoStartRegistrySandbox sandbox(L"persisted-true");
+    const auto path = test_state_path("persisted-true");
+    desktopgrass::persistence::SetStateFilePathForTest(path.wstring());
+
+    desktopgrass::persistence::AppState state;
+    state.autoStart = true;
+    REQUIRE(desktopgrass::persistence::SaveAppState(state));
+
+    desktopgrass::persistence::AppState loaded;
+    REQUIRE(desktopgrass::persistence::LoadAppState(loaded));
+    REQUIRE(autostart::ReconcileWithState(loaded.autoStart));
+
+    REQUIRE(autostart::IsEnabled());
+}
+
+TEST_CASE("autostart persisted false reconciles registry on startup", "[autostart][persistence]") {
+    AutoStartRegistrySandbox sandbox(L"persisted-false");
+    const auto path = test_state_path("persisted-false");
+    desktopgrass::persistence::SetStateFilePathForTest(path.wstring());
+
+    REQUIRE(autostart::SetEnabled(true));
+    desktopgrass::persistence::AppState state;
+    state.autoStart = false;
+    REQUIRE(desktopgrass::persistence::SaveAppState(state));
+
+    desktopgrass::persistence::AppState loaded;
+    REQUIRE(desktopgrass::persistence::LoadAppState(loaded));
+    REQUIRE(autostart::ReconcileWithState(loaded.autoStart));
+
+    REQUIRE_FALSE(autostart::IsEnabled());
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/autumn_tests.cpp

@@ -0,0 +1,600 @@
+// autumn_tests.cpp
+//
+// Autumn scene tests (architecture.md §16.5).
+
+#include "../third_party/catch2/catch.hpp"
+#include "Persistence.h"
+#include "Sim.h"
+#include "snapshot_data.h"
+
+#include <algorithm>
+#include <array>
+#include <cmath>
+#include <filesystem>
+
+using namespace desktopgrass;
+
+namespace {
+
+constexpr double kMonitor1920 = 1920.0;
+constexpr double kEpsilon = 1e-9;
+constexpr double kTwoPi = 6.28318530717958647692;
+
+Sim make_sim(uint64_t seed = CANONICAL_TEST_SEED,
+             double width = kMonitor1920,
+             double density = DEFAULT_DENSITY) {
+    return sim_init(seed, width, density);
+}
+
+Sim make_autumn_sim(uint64_t seed = CANONICAL_TEST_SEED,
+                    double width = kMonitor1920,
+                    double density = DEFAULT_DENSITY) {
+    Sim sim = make_sim(seed, width, density);
+    sim_set_scene(sim, Scene::Autumn);
+    return sim;
+}
+
+int count_kind(const Sim& sim, EntityKind kind) {
+    return static_cast<int>(std::count_if(sim.entities.begin(), sim.entities.end(),
+        [kind](const Entity& e) { return e.kind == kind; }));
+}
+
+int count_maples(const Sim& sim) {
+    return static_cast<int>(std::count_if(sim.blades.begin(), sim.blades.end(),
+        [](const Blade& b) { return b.isMaple; }));
+}
+
+int count_new_leaf_spawns(Sim& sim, double seconds, double dt = 0.05) {
+    int count = 0;
+    const int steps = static_cast<int>(std::ceil(seconds / dt));
+    for (int i = 0; i < steps; ++i) {
+        sim_tick(sim, dt, nullptr, 0);
+        for (const Entity& e : sim.entities) {
+            if (e.kind == EntityKind::Leaf && e.age == Approx(0.0).margin(kEpsilon)) {
+                ++count;
+            }
+        }
+    }
+    return count;
+}
+
+const Entity& spawn_next_leaf(Sim& sim) {
+    const double dt = std::max(0.0, sim.nextLeafSpawnTime - sim.globalTime);
+    sim_tick(sim, dt, nullptr, 0);
+    auto it = std::find_if(sim.entities.rbegin(), sim.entities.rend(),
+        [](const Entity& e) { return e.kind == EntityKind::Leaf && e.age == Approx(0.0).margin(kEpsilon); });
+    REQUIRE(it != sim.entities.rend());
+    return *it;
+}
+
+const Blade* first_maple(const Sim& sim) {
+    auto it = std::find_if(sim.blades.begin(), sim.blades.end(),
+        [](const Blade& b) { return b.isMaple; });
+    return it == sim.blades.end() ? nullptr : &*it;
+}
+
+Sim make_autumn_sim_with_maple(uint64_t* outSeed = nullptr) {
+    for (uint64_t offset = 0; offset < 512; ++offset) {
+        const uint64_t seed = CANONICAL_TEST_SEED + offset;
+        Sim sim = make_autumn_sim(seed);
+        if (count_maples(sim) > 0) {
+            if (outSeed) *outSeed = seed;
+            return sim;
+        }
+    }
+    FAIL("Unable to find deterministic seed with a maple");
+    return make_autumn_sim();
+}
+
+// Find an Autumn sim that contains at least one leafy (non-bare) maple, and
+// return a pointer to it. The returned pointer is valid for the lifetime of
+// the returned-by-out sim.
+inline const Blade* first_leafy_maple(const Sim& sim) {
+    auto it = std::find_if(sim.blades.begin(), sim.blades.end(),
+        [](const Blade& b) { return b.isMaple && !b.mapleIsBare; });
+    return it == sim.blades.end() ? nullptr : &*it;
+}
+
+Sim make_autumn_sim_with_leafy_maple() {
+    for (uint64_t offset = 0; offset < 2048; ++offset) {
+        Sim sim = make_autumn_sim(CANONICAL_TEST_SEED + offset);
+        if (first_leafy_maple(sim) != nullptr) return sim;
+    }
+    FAIL("Unable to find deterministic seed with a leafy maple");
+    return make_autumn_sim();
+}
+
+std::filesystem::path autumn_state_path() {
+    std::filesystem::path dir = std::filesystem::current_path()
+        / ".copilot-scratch"
+        / "native-autumn-tests";
+    std::error_code ec;
+    std::filesystem::remove_all(dir, ec);
+    std::filesystem::create_directories(dir);
+    return dir / "state.json";
+}
+
+} // namespace
+
+TEST_CASE("Autumn scene count bumps to five", "[autumn][scene]") {
+    REQUIRE(SCENE_COUNT == 5);
+}
+
+TEST_CASE("Autumn scene enum value is pinned", "[autumn][scene]") {
+    REQUIRE(static_cast<int>(Scene::Autumn) == 3);
+}
+
+TEST_CASE("Autumn palette is pinned in scene palettes", "[autumn][palette]") {
+    for (int i = 0; i < PALETTE_SIZE; ++i) {
+        REQUIRE(SCENE_PALETTES[static_cast<int>(Scene::Autumn)][i] == AUTUMN_PALETTE[i]);
+    }
+}
+
+TEST_CASE("Autumn does not change default scene", "[autumn][scene]") {
+    REQUIRE(SCENE_DEFAULT == Scene::Grass);
+}
+
+TEST_CASE("Leaf constants match Autumn spec", "[autumn][leaf][constants]") {
+    REQUIRE(LEAF_SPAWN_RATE_PER_SEC_1920DIP == Approx(1.4));
+    REQUIRE(LEAF_FALL_SPEED_MIN == Approx(14.0));
+    REQUIRE(LEAF_FALL_SPEED_MAX == Approx(26.0));
+    REQUIRE(LEAF_HORIZONTAL_DRIFT_AMP == Approx(32.0));
+    REQUIRE(LEAF_HORIZONTAL_DRIFT_FREQ == Approx(1.4));
+    REQUIRE(LEAF_ROTATION_SPEED_MIN == Approx(0.8));
+    REQUIRE(LEAF_ROTATION_SPEED_MAX == Approx(2.4));
+    REQUIRE(LEAF_SIZE_MIN == Approx(4.0));
+    REQUIRE(LEAF_SIZE_MAX == Approx(7.0));
+    REQUIRE(LEAF_SPAWN_Y_OFFSET == Approx(-10.0));
+    REQUIRE(LEAF_COLOR_COUNT == 6);
+    constexpr uint32_t expected[LEAF_COLOR_COUNT] = {
+        0xFFD96B0Cu, 0xFFB54D1Eu, 0xFFE89A3Cu,
+        0xFFC23E12u, 0xFFE6C849u, 0xFF8C2E0Fu,
+    };
+    for (int i = 0; i < LEAF_COLOR_COUNT; ++i) {
+        REQUIRE(LEAF_COLORS[i] == expected[i]);
+    }
+    REQUIRE(LEAF_PRNG_SALT == 0x1EA1DEC1D1EA1D05ull);
+}
+
+TEST_CASE("Autumn leaf spawn rate is gated and near mean", "[autumn][leaf]") {
+    Sim autumn = make_autumn_sim();
+    const int count = count_new_leaf_spawns(autumn, 100.0);
+    REQUIRE(count >= 112);
+    REQUIRE(count <= 168);
+}
+
+TEST_CASE("Only Autumn spawns leaves", "[autumn][leaf][gating]") {
+    for (Scene scene : { Scene::Grass, Scene::Desert, Scene::Winter }) {
+        Sim sim = make_sim();
+        sim_set_scene(sim, scene);
+        count_new_leaf_spawns(sim, 30.0);
+        REQUIRE(count_kind(sim, EntityKind::Leaf) == 0);
+    }
+}
+
+TEST_CASE("Leaf fall speed stays within pinned range", "[autumn][leaf]") {
+    Sim sim = make_autumn_sim();
+    for (int i = 0; i < 32; ++i) {
+        const Entity& e = spawn_next_leaf(sim);
+        REQUIRE(e.vy >= LEAF_FALL_SPEED_MIN);
+        REQUIRE(e.vy <= LEAF_FALL_SPEED_MAX);
+        REQUIRE(e.baseSpeed == Approx(e.vy));
+    }
+}
+
+TEST_CASE("Leaf size stays within pinned range", "[autumn][leaf]") {
+    Sim sim = make_autumn_sim();
+    for (int i = 0; i < 32; ++i) {
+        const Entity& e = spawn_next_leaf(sim);
+        REQUIRE(e.size >= LEAF_SIZE_MIN);
+        REQUIRE(e.size <= LEAF_SIZE_MAX);
+    }
+}
+
+TEST_CASE("Leaf color variant stays within pinned range", "[autumn][leaf]") {
+    Sim sim = make_autumn_sim();
+    for (int i = 0; i < 32; ++i) {
+        const Entity& e = spawn_next_leaf(sim);
+        REQUIRE(e.colorVariant < LEAF_COLOR_COUNT);
+    }
+}
+
+TEST_CASE("Leaf PRNG draw order matches side stream", "[autumn][leaf][prng]") {
+    Sim sim = make_autumn_sim();
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ LEAF_PRNG_SALT);
+    const double lambda = LEAF_SPAWN_RATE_PER_SEC_1920DIP * sim.monitorWidth / 1920.0;
+    double expectedNext = 0.0;
+
+    for (int i = 0; i < 8; ++i) {
+        const Entity& e = spawn_next_leaf(sim);
+        const double xFrac = prng_uniform(side, 0.0, 1.0);
+        const double expectedSpawnX = xFrac * sim.monitorWidth;
+        const double expectedFallSpeed = prng_uniform(side, LEAF_FALL_SPEED_MIN, LEAF_FALL_SPEED_MAX);
+        const double expectedPhase = prng_uniform(side, 0.0, kTwoPi);
+        const double rotationMag = prng_uniform(side, LEAF_ROTATION_SPEED_MIN, LEAF_ROTATION_SPEED_MAX);
+        const double rotationSign = (prng_next_u64(side) & 1ull) != 0ull ? 1.0 : -1.0;
+        const double expectedRotation = prng_uniform(side, 0.0, kTwoPi);
+        const double expectedSize = prng_uniform(side, LEAF_SIZE_MIN, LEAF_SIZE_MAX);
+        const uint8_t expectedColor = static_cast<uint8_t>(prng_index(side, LEAF_COLOR_COUNT));
+        expectedNext += prng_exponential(side, lambda);
+
+        REQUIRE(e.x0 == Approx(expectedSpawnX).margin(kEpsilon));
+        REQUIRE(e.x == Approx(expectedSpawnX + LEAF_HORIZONTAL_DRIFT_AMP * std::sin(expectedPhase)).margin(kEpsilon));
+        REQUIRE(e.vy == Approx(expectedFallSpeed).margin(kEpsilon));
+        REQUIRE(e.phaseX == Approx(expectedPhase).margin(kEpsilon));
+        REQUIRE(e.rotationSpeed == Approx(rotationMag * rotationSign).margin(kEpsilon));
+        REQUIRE(e.rotation == Approx(expectedRotation).margin(kEpsilon));
+        REQUIRE(e.size == Approx(expectedSize).margin(kEpsilon));
+        REQUIRE(e.colorVariant == expectedColor);
+        REQUIRE(sim.nextLeafSpawnTime == Approx(expectedNext).margin(kEpsilon));
+    }
+}
+
+TEST_CASE("Leaf despawns when past ground", "[autumn][leaf]") {
+    Sim sim = make_autumn_sim();
+    Entity e{};
+    e.kind = EntityKind::Leaf;
+    e.y = sim.windowHeight + 0.1;
+    e.lifetime = -1.0;
+    sim.entities.push_back(e);
+    sim.nextLeafSpawnTime = 1.0e9;
+    sim_tick_entities(sim, 0.0);
+    REQUIRE(count_kind(sim, EntityKind::Leaf) == 0);
+}
+
+TEST_CASE("Leaf ignores click-cut interaction", "[autumn][leaf]") {
+    Sim sim = make_autumn_sim();
+    Entity e{};
+    e.kind = EntityKind::Leaf;
+    e.x = 200.0;
+    e.y = sim.windowHeight - 5.0;
+    e.size = 5.0;
+    e.lifetime = -1.0;
+    sim.entities.push_back(e);
+
+    InputEvent click{};
+    click.type = EventType::Click;
+    click.x = e.x;
+    click.y = e.y;
+    click.time = sim.globalTime;
+    sim_apply_click(sim, click);
+
+    REQUIRE(count_kind(sim, EntityKind::Leaf) == 1);
+}
+
+TEST_CASE("Leaf EntityKind value is pinned", "[autumn][leaf][enum]") {
+    REQUIRE(static_cast<int>(EntityKind::Leaf) == 11);
+}
+
+TEST_CASE("Maple constants match Autumn spec", "[autumn][maple][constants]") {
+    REQUIRE(MAPLE_PROBABILITY == Approx(0.0070));
+    REQUIRE(MAPLE_HEIGHT_MIN == Approx(50.0));
+    REQUIRE(MAPLE_HEIGHT_MAX == Approx(85.0));
+    REQUIRE(MAPLE_TRUNK_WIDTH_MIN == Approx(6.0));
+    REQUIRE(MAPLE_TRUNK_WIDTH_MAX == Approx(10.0));
+    REQUIRE(MAPLE_CANOPY_RADIUS_MIN == Approx(14.0));
+    REQUIRE(MAPLE_CANOPY_RADIUS_MAX == Approx(24.0));
+    REQUIRE(MAPLE_TRUNK_COLOR == 0xFF4A2C18u);
+    REQUIRE(MAPLE_TRUNK_DARK == 0xFF2F1B0Eu);
+    REQUIRE(MAPLE_CANOPY_COLOR_COUNT == 4);
+    constexpr uint32_t expected[MAPLE_CANOPY_COLOR_COUNT] = {
+        0xFFD96B0Cu, 0xFFE89A3Cu, 0xFFC23E12u, 0xFFE6C849u,
+    };
+    for (int i = 0; i < MAPLE_CANOPY_COLOR_COUNT; ++i) {
+        REQUIRE(MAPLE_CANOPY_COLORS[i] == expected[i]);
+    }
+    REQUIRE(MAPLE_BARE_FRACTION == Approx(0.20));
+    REQUIRE(MAPLE_PRNG_SALT == 0xC1AA51EC1AA51Eull);
+}
+
+TEST_CASE("Maples generate only in Autumn", "[autumn][maple][gating]") {
+    for (Scene scene : { Scene::Grass, Scene::Desert, Scene::Winter }) {
+        Sim sim = make_sim();
+        sim_set_scene(sim, scene);
+        REQUIRE(count_maples(sim) == 0);
+    }
+    REQUIRE(count_maples(make_autumn_sim_with_maple()) > 0);
+}
+
+TEST_CASE("Maple promotion probability is near spec", "[autumn][maple]") {
+    int totalSlots = 0;
+    int totalMaples = 0;
+    for (uint64_t seed = CANONICAL_TEST_SEED; seed < CANONICAL_TEST_SEED + 200; ++seed) {
+        Sim sim = make_autumn_sim(seed);
+        totalSlots += static_cast<int>(sim.blades.size());
+        totalMaples += count_maples(sim);
+    }
+    const double fraction = static_cast<double>(totalMaples) / static_cast<double>(totalSlots);
+    REQUIRE(fraction >= MAPLE_PROBABILITY * 0.75);
+    REQUIRE(fraction <= MAPLE_PROBABILITY * 1.25);
+}
+
+TEST_CASE("Maple height stays within pinned range", "[autumn][maple]") {
+    Sim sim = make_autumn_sim_with_maple();
+    for (const Blade& b : sim.blades) if (b.isMaple) {
+        REQUIRE(b.mapleHeight >= MAPLE_HEIGHT_MIN);
+        REQUIRE(b.mapleHeight <= MAPLE_HEIGHT_MAX);
+    }
+}
+
+TEST_CASE("Maple trunk width stays within pinned range", "[autumn][maple]") {
+    Sim sim = make_autumn_sim_with_maple();
+    for (const Blade& b : sim.blades) if (b.isMaple) {
+        REQUIRE(b.mapleTrunkWidth >= MAPLE_TRUNK_WIDTH_MIN);
+        REQUIRE(b.mapleTrunkWidth <= MAPLE_TRUNK_WIDTH_MAX);
+    }
+}
+
+TEST_CASE("Maple canopy radius stays within pinned range", "[autumn][maple]") {
+    Sim sim = make_autumn_sim_with_maple();
+    for (const Blade& b : sim.blades) if (b.isMaple) {
+        REQUIRE(b.mapleCanopyRadius >= MAPLE_CANOPY_RADIUS_MIN);
+        REQUIRE(b.mapleCanopyRadius <= MAPLE_CANOPY_RADIUS_MAX);
+    }
+}
+
+TEST_CASE("Maple canopy color variant stays within pinned range", "[autumn][maple]") {
+    Sim sim = make_autumn_sim_with_maple();
+    for (const Blade& b : sim.blades) if (b.isMaple) {
+        REQUIRE(b.mapleCanopyColorIdx < MAPLE_CANOPY_COLOR_COUNT);
+    }
+}
+
+TEST_CASE("Maple bare fraction is near spec", "[autumn][maple]") {
+    int totalMaples = 0;
+    int totalBare = 0;
+    for (uint64_t seed = CANONICAL_TEST_SEED; seed < CANONICAL_TEST_SEED + 400; ++seed) {
+        Sim sim = make_autumn_sim(seed);
+        for (const Blade& b : sim.blades) if (b.isMaple) {
+            ++totalMaples;
+            if (b.mapleIsBare) ++totalBare;
+        }
+    }
+    REQUIRE(totalMaples > 100);
+    const double fraction = static_cast<double>(totalBare) / static_cast<double>(totalMaples);
+    REQUIRE(fraction >= MAPLE_BARE_FRACTION - 0.05);
+    REQUIRE(fraction <= MAPLE_BARE_FRACTION + 0.05);
+}
+
+TEST_CASE("Maple PRNG draw order matches side stream", "[autumn][maple][prng]") {
+    uint64_t seed = 0;
+    Sim sim = make_autumn_sim_with_maple(&seed);
+    Prng side;
+    prng_init(side, seed ^ MAPLE_PRNG_SALT);
+
+    for (std::size_t i = 0; i < sim.blades.size(); ++i) {
+        const double r = prng_uniform(side, 0.0, 1.0);
+        if (r >= MAPLE_PROBABILITY) {
+            REQUIRE_FALSE(sim.blades[i].isMaple);
+            continue;
+        }
+
+        const double expectedHeight = prng_uniform(side, MAPLE_HEIGHT_MIN, MAPLE_HEIGHT_MAX);
+        const double expectedTrunkWidth = prng_uniform(side, MAPLE_TRUNK_WIDTH_MIN, MAPLE_TRUNK_WIDTH_MAX);
+        const double expectedCanopyRadius = prng_uniform(side, MAPLE_CANOPY_RADIUS_MIN, MAPLE_CANOPY_RADIUS_MAX);
+        const uint8_t expectedColor = static_cast<uint8_t>(prng_index(side, MAPLE_CANOPY_COLOR_COUNT));
+        const bool expectedBare = prng_uniform(side, 0.0, 1.0) < MAPLE_BARE_FRACTION;
+
+        const Blade& b = sim.blades[i];
+        REQUIRE(b.isMaple);
+        REQUIRE(b.mapleHeight == Approx(expectedHeight).margin(kEpsilon));
+        REQUIRE(b.mapleTrunkWidth == Approx(expectedTrunkWidth).margin(kEpsilon));
+        REQUIRE(b.mapleCanopyRadius == Approx(expectedCanopyRadius).margin(kEpsilon));
+        REQUIRE(b.mapleCanopyColorIdx == expectedColor);
+        REQUIRE(b.mapleIsBare == expectedBare);
+        return;
+    }
+    FAIL("Expected a maple promotion");
+}
+
+TEST_CASE("Maples are cuttable through existing cut model", "[autumn][maple]") {
+    Sim sim = make_autumn_sim_with_maple();
+    const Blade* maple = first_maple(sim);
+    REQUIRE(maple != nullptr);
+    const double clickX = maple->baseX;
+
+    InputEvent click{};
+    click.type = EventType::Click;
+    click.x = clickX;
+    click.y = sim.windowHeight - 1.0;
+    click.time = sim.globalTime;
+    sim_apply_click(sim, click);
+    sim_tick(sim, CUT_DURATION_SEC + 0.01, nullptr, 0);
+
+    const Blade& cutMaple = *std::find_if(sim.blades.begin(), sim.blades.end(),
+        [clickX](const Blade& b) { return b.isMaple && b.baseX == Approx(clickX); });
+    // Cut blades now settle at their per-blade stubble floor, not flat zero.
+    REQUIRE(cutMaple.cutFloor > 0.0);
+    REQUIRE(cutMaple.cutHeight == Approx(cutMaple.cutFloor).margin(kEpsilon));
+}
+
+TEST_CASE("Autumn is critter-free", "[autumn][critter][gating]") {
+    Sim sim = make_autumn_sim();
+    REQUIRE(count_kind(sim, EntityKind::Sheep) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Bunny) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Hedgehog) == 0);
+}
+
+
+TEST_CASE("Autumn does not spawn snowflakes", "[autumn][weather]") {
+    Sim sim = make_autumn_sim();
+    for (int i = 0; i < 500; ++i) sim_tick(sim, 0.05, nullptr, 0);
+    REQUIRE(count_kind(sim, EntityKind::Snowflake) == 0);
+}
+
+TEST_CASE("Autumn scene persists round-trip", "[autumn][persistence]") {
+    const auto path = autumn_state_path();
+    persistence::SetStateFilePathForTest(path.wstring());
+
+    persistence::AppState expected;
+    expected.scene = Scene::Autumn;
+    REQUIRE(persistence::SaveAppState(expected));
+
+    persistence::AppState actual;
+    REQUIRE(persistence::LoadAppState(actual));
+    REQUIRE(actual.scene == Scene::Autumn);
+}
+
+TEST_CASE("Leaf puff constants are pinned", "[autumn][leaf][puff][constants]") {
+    REQUIRE(LEAF_PUFF_COUNT_MIN == 4);
+    REQUIRE(LEAF_PUFF_COUNT_MAX == 7);
+    REQUIRE(LEAF_PUFF_BURST_SPEED_MIN == Approx(18.0));
+    REQUIRE(LEAF_PUFF_BURST_SPEED_MAX == Approx(42.0));
+    REQUIRE(LEAF_PUFF_DRAG == Approx(2.2));
+    REQUIRE(LEAF_PUFF_COOLDOWN_SEC == Approx(1.5));
+    REQUIRE(LEAF_PUFF_HOVER_RADIUS_MUL == Approx(1.15));
+    REQUIRE(LEAF_PUFF_MIN_CUT_HEIGHT == Approx(0.5));
+    REQUIRE(LEAF_PUFF_START_OFFSET_FRAC == Approx(0.4));
+}
+
+TEST_CASE("Hovering a leafy maple sheds a leaf puff", "[autumn][leaf][puff]") {
+    Sim sim = make_autumn_sim_with_leafy_maple();
+    const Blade* maple = first_leafy_maple(sim);
+    REQUIRE(maple != nullptr);
+    const double cx = maple->baseX;
+    const double cy = sim.windowHeight - maple->mapleHeight * maple->cutHeight;
+
+    const int before = count_kind(sim, EntityKind::Leaf);
+    InputEvent mv{};
+    mv.type = EventType::Move;
+    mv.x = cx;
+    mv.y = cy;
+    mv.time = sim.globalTime;
+    sim_apply_move(sim, mv);
+
+    const int puffed = count_kind(sim, EntityKind::Leaf) - before;
+    REQUIRE(puffed >= LEAF_PUFF_COUNT_MIN);
+    REQUIRE(puffed <= LEAF_PUFF_COUNT_MAX);
+
+    const bool anyBurst = std::any_of(sim.entities.begin(), sim.entities.end(),
+        [](const Entity& e) { return e.kind == EntityKind::Leaf && e.vx != 0.0; });
+    REQUIRE(anyBurst);
+}
+
+TEST_CASE("Leaf puff respects a per-tree cooldown", "[autumn][leaf][puff]") {
+    Sim sim = make_autumn_sim_with_leafy_maple();
+    const Blade* maple = first_leafy_maple(sim);
+    const double cx = maple->baseX;
+    const double cy = sim.windowHeight - maple->mapleHeight * maple->cutHeight;
+
+    InputEvent mv{};
+    mv.type = EventType::Move;
+    mv.x = cx;
+    mv.y = cy;
+    mv.time = sim.globalTime;
+    sim_apply_move(sim, mv);
+    const int afterFirst = count_kind(sim, EntityKind::Leaf);
+    REQUIRE(afterFirst > 0);
+
+    sim_apply_move(sim, mv);
+    REQUIRE(count_kind(sim, EntityKind::Leaf) == afterFirst);
+
+    sim.globalTime += LEAF_PUFF_COOLDOWN_SEC + 0.1;
+    mv.time = sim.globalTime;
+    sim_apply_move(sim, mv);
+    REQUIRE(count_kind(sim, EntityKind::Leaf) > afterFirst);
+}
+
+TEST_CASE("Leaf puff ignores cursor away from canopy", "[autumn][leaf][puff]") {
+    Sim sim = make_autumn_sim_with_leafy_maple();
+    const Blade* maple = first_leafy_maple(sim);
+    const int before = count_kind(sim, EntityKind::Leaf);
+
+    InputEvent mv{};
+    mv.type = EventType::Move;
+    mv.x = maple->baseX + 400.0;
+    mv.y = sim.windowHeight - maple->mapleHeight * maple->cutHeight;
+    mv.time = sim.globalTime;
+    sim_apply_move(sim, mv);
+    REQUIRE(count_kind(sim, EntityKind::Leaf) == before);
+}
+
+TEST_CASE("Leaf puff does not fire outside Autumn", "[autumn][leaf][puff][gating]") {
+    Sim sim = make_autumn_sim_with_leafy_maple();
+    const Blade* maple = first_leafy_maple(sim);
+    const double cx = maple->baseX;
+    const double cy = sim.windowHeight - maple->mapleHeight * maple->cutHeight;
+    sim.currentScene = Scene::Grass;
+
+    const int before = count_kind(sim, EntityKind::Leaf);
+    InputEvent mv{};
+    mv.type = EventType::Move;
+    mv.x = cx;
+    mv.y = cy;
+    mv.time = sim.globalTime;
+    sim_apply_move(sim, mv);
+    REQUIRE(count_kind(sim, EntityKind::Leaf) == before);
+}
+
+TEST_CASE("Puff burst decays so leaves settle into flutter", "[autumn][leaf][puff]") {
+    Sim sim = make_autumn_sim_with_leafy_maple();
+    const Blade* maple = first_leafy_maple(sim);
+    InputEvent mv{};
+    mv.type = EventType::Move;
+    mv.x = maple->baseX;
+    mv.y = sim.windowHeight - maple->mapleHeight * maple->cutHeight;
+    mv.time = sim.globalTime;
+    sim_apply_move(sim, mv);
+    REQUIRE(count_kind(sim, EntityKind::Leaf) > 0);
+
+    for (int i = 0; i < 40; ++i) sim_tick(sim, 0.05, nullptr, 0);
+    for (const Entity& e : sim.entities) {
+        if (e.kind == EntityKind::Leaf) REQUIRE(e.vx == Approx(0.0).margin(kEpsilon));
+    }
+}
+
+TEST_CASE("Re-entering Autumn clears the puff cooldown", "[autumn][leaf][puff]") {
+    Sim sim = make_autumn_sim_with_leafy_maple();
+    const Blade* maple = first_leafy_maple(sim);
+    InputEvent mv{};
+    mv.type = EventType::Move;
+    mv.x = maple->baseX;
+    mv.y = sim.windowHeight - maple->mapleHeight * maple->cutHeight;
+    mv.time = sim.globalTime;
+    sim_apply_move(sim, mv);
+    REQUIRE(count_kind(sim, EntityKind::Leaf) > 0);
+
+    // Leaving and re-entering Autumn regenerates the (deterministic) maples and
+    // must reset their puff cooldown so the fresh scene can puff immediately.
+    sim_set_scene(sim, Scene::Grass);
+    sim_set_scene(sim, Scene::Autumn);
+    const Blade* maple2 = first_leafy_maple(sim);
+    REQUIRE(maple2 != nullptr);
+
+    const int before = count_kind(sim, EntityKind::Leaf);
+    InputEvent mv2{};
+    mv2.type = EventType::Move;
+    mv2.x = maple2->baseX;
+    mv2.y = sim.windowHeight - maple2->mapleHeight * maple2->cutHeight;
+    mv2.time = sim.globalTime;
+    sim_apply_move(sim, mv2);
+    REQUIRE(count_kind(sim, EntityKind::Leaf) > before);
+}
+
+TEST_CASE("Autumn PRNG salts are unique", "[autumn][prng]") {
+    constexpr std::array<uint64_t, 16> salts = {
+        REGROW_PRNG_SALT,
+        FLOWER_PRNG_SALT,
+        MUSHROOM_PRNG_SALT,
+        AMBIENT_GUST_PRNG_SALT,
+        CACTUS_PRNG_SALT,
+        TUMBLEWEED_PRNG_SALT,
+        CRITTER_PRNG_SALT,
+        BUTTERFLY_PRNG_SALT,
+        FIREFLY_PRNG_SALT,
+        BIRD_FLYBY_PRNG_SALT,
+        SNOWFLAKE_PRNG_SALT,
+        PINE_PRNG_SALT,
+        LEAF_PRNG_SALT,
+        MAPLE_PRNG_SALT,
+        LEAF_PUFF_PRNG_SALT,
+    };
+
+    for (std::size_t i = 0; i < salts.size(); ++i) {
+        for (std::size_t j = i + 1; j < salts.size(); ++j) {
+            REQUIRE(salts[i] != salts[j]);
+        }
+    }
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/bird_flyby_tests.cpp

@@ -0,0 +1,365 @@
+// bird_flyby_tests.cpp - §17.8 ambient bird flyby tests.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+#include <cmath>
+#include <vector>
+
+using namespace desktopgrass;
+
+namespace {
+constexpr double Monitor1920 = 1920.0;
+constexpr double TwoPi = 6.28318530717958647692;
+
+Sim build_sim(uint64_t seed = CANONICAL_TEST_SEED) {
+    Sim sim = sim_init(seed, Monitor1920, DEFAULT_DENSITY);
+    sim.currentScene = Scene::Grass;
+    sim.entities.clear();
+    return sim;
+}
+
+int count_birds(const Sim& sim) {
+    return static_cast<int>(std::count_if(sim.entities.begin(), sim.entities.end(),
+        [](const Entity& e) { return e.kind == EntityKind::Bird; }));
+}
+
+std::vector<Entity> birds(const Sim& sim) {
+    std::vector<Entity> out;
+    for (const Entity& e : sim.entities) {
+        if (e.kind == EntityKind::Bird) out.push_back(e);
+    }
+    return out;
+}
+
+int prng_count(Prng& side, int minCount, int maxCount) {
+    const double draw = prng_uniform(side, static_cast<double>(minCount), static_cast<double>(maxCount + 1));
+    int count = static_cast<int>(std::floor(draw));
+    if (count < minCount) count = minCount;
+    if (count > maxCount) count = maxCount;
+    return count;
+}
+
+void reset_bird_stream_fresh(Sim& sim, uint64_t seed) {
+    prng_init(sim.birdFlybyPrng, seed ^ BIRD_FLYBY_PRNG_SALT);
+    sim.nextBirdFlybyAtTime = sim.globalTime;
+}
+
+void reset_bird_schedule(Sim& sim, uint64_t seed) {
+    prng_init(sim.birdFlybyPrng, seed ^ BIRD_FLYBY_PRNG_SALT);
+    sim.nextBirdFlybyAtTime = sim.globalTime + bird_flyby_sample_interval(sim.birdFlybyPrng);
+}
+
+uint64_t find_seed_for_flock_size(int size) {
+    for (uint64_t i = 1; i < 10000; ++i) {
+        Sim sim = build_sim(CANONICAL_TEST_SEED + i);
+        reset_bird_stream_fresh(sim, CANONICAL_TEST_SEED + i);
+        sim_spawn_bird_flyby(sim);
+        if (count_birds(sim) == size) return CANONICAL_TEST_SEED + i;
+    }
+    return CANONICAL_TEST_SEED;
+}
+
+uint64_t find_v_seed(int minSize) {
+    for (uint64_t i = 1; i < 10000; ++i) {
+        Sim sim = build_sim(CANONICAL_TEST_SEED + i);
+        reset_bird_stream_fresh(sim, CANONICAL_TEST_SEED + i);
+        sim_spawn_bird_flyby(sim);
+        auto flock = birds(sim);
+        if (static_cast<int>(flock.size()) >= minSize && flock[0].colorVariant == 0) {
+            return CANONICAL_TEST_SEED + i;
+        }
+    }
+    return CANONICAL_TEST_SEED;
+}
+} // namespace
+
+TEST_CASE("Bird flyby constants are pinned to spec values", "[bird][constants]") {
+    REQUIRE(BIRD_FLYBY_SPAWN_RATE_PER_HOUR == Approx(15.0));
+    REQUIRE(BIRD_FLOCK_SIZE_MIN == 3);
+    REQUIRE(BIRD_FLOCK_SIZE_MAX == 7);
+    REQUIRE(BIRD_FLOCK_FORMATION_SPACING == Approx(9.0));
+    REQUIRE(BIRD_FLOCK_V_ANGLE_DEG == Approx(22.0));
+    REQUIRE(BIRD_SPEED_MIN == Approx(65.0));
+    REQUIRE(BIRD_SPEED_MAX == Approx(95.0));
+    REQUIRE(BIRD_ALTITUDE_MIN == Approx(78.0));
+    REQUIRE(BIRD_ALTITUDE_MAX == Approx(96.0));
+    REQUIRE(BIRD_BODY_LENGTH == Approx(3.6));
+    REQUIRE(BIRD_WING_SPAN == Approx(5.0));
+    REQUIRE(BIRD_WING_FLAP_FREQ == Approx(7.0));
+    REQUIRE(BIRD_WING_FLAP_PHASE_JITTER == Approx(0.6));
+    REQUIRE(BIRD_BODY_COLOR == 0xFF1A1610u);
+    REQUIRE(BIRD_WING_OPEN_RATIO == Approx(1.0));
+    REQUIRE(BIRD_WING_FOLD_RATIO == Approx(0.30));
+    REQUIRE(BIRD_FADE_IN_FRAC == Approx(0.08));
+    REQUIRE(BIRD_FADE_OUT_FRAC == Approx(0.08));
+    REQUIRE(BIRD_DRIFT_AMP_Y == Approx(3.0));
+    REQUIRE(BIRD_DRIFT_FREQ_Y == Approx(0.8));
+    REQUIRE(BIRD_FLYBY_PRNG_SALT == 0xB12D1F1A1B12D1Aull);
+}
+
+TEST_CASE("Bird flyby PRNG salt is unique", "[bird][constants]") {
+    const uint64_t salts[] = {
+        REGROW_PRNG_SALT, FLOWER_PRNG_SALT, MUSHROOM_PRNG_SALT,
+        AMBIENT_GUST_PRNG_SALT, CACTUS_PRNG_SALT, TUMBLEWEED_PRNG_SALT,
+        CRITTER_PRNG_SALT, BUTTERFLY_PRNG_SALT, FIREFLY_PRNG_SALT,
+        SNOWFLAKE_PRNG_SALT, PINE_PRNG_SALT,
+    };
+    for (uint64_t salt : salts) {
+        REQUIRE(BIRD_FLYBY_PRNG_SALT != salt);
+    }
+}
+
+TEST_CASE("Bird flyby flock size stays in range over seeds", "[bird][spawn]") {
+    for (uint64_t i = 0; i < 256; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i * 0x9E3779B97F4A7C15ull;
+        Sim sim = build_sim(seed);
+        reset_bird_stream_fresh(sim, seed);
+        sim_spawn_bird_flyby(sim);
+        REQUIRE(count_birds(sim) >= BIRD_FLOCK_SIZE_MIN);
+        REQUIRE(count_birds(sim) <= BIRD_FLOCK_SIZE_MAX);
+    }
+}
+
+TEST_CASE("Bird flyby leader altitude stays in range", "[bird][spawn]") {
+    for (uint64_t i = 0; i < 128; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i;
+        Sim sim = build_sim(seed);
+        reset_bird_stream_fresh(sim, seed);
+        sim_spawn_bird_flyby(sim);
+        auto flock = birds(sim);
+        REQUIRE(!flock.empty());
+        REQUIRE(flock[0].altitudeAnchor >= BIRD_ALTITUDE_MIN);
+        REQUIRE(flock[0].altitudeAnchor <  BIRD_ALTITUDE_MAX);
+    }
+}
+
+TEST_CASE("Bird flyby leader speed stays in range", "[bird][spawn]") {
+    for (uint64_t i = 0; i < 128; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i;
+        Sim sim = build_sim(seed);
+        reset_bird_stream_fresh(sim, seed);
+        sim_spawn_bird_flyby(sim);
+        auto flock = birds(sim);
+        REQUIRE(!flock.empty());
+        REQUIRE(flock[0].baseSpeed >= BIRD_SPEED_MIN);
+        REQUIRE(flock[0].baseSpeed <  BIRD_SPEED_MAX);
+        REQUIRE(std::abs(flock[0].vx) == Approx(flock[0].baseSpeed));
+    }
+}
+
+TEST_CASE("Bird flyby PRNG draw order matches side stream", "[bird][prng]") {
+    const uint64_t seed = 0xB17D5EED1234ull;
+    Sim sim = build_sim(seed);
+    reset_bird_stream_fresh(sim, seed);
+    Prng side;
+    prng_init(side, seed ^ BIRD_FLYBY_PRNG_SALT);
+
+    sim_spawn_bird_flyby(sim);
+    const int expectedCount = prng_count(side, BIRD_FLOCK_SIZE_MIN, BIRD_FLOCK_SIZE_MAX);
+    const uint64_t directionBit = prng_next_u64(side) & 1ull;
+    const double direction = directionBit != 0ull ? 1.0 : -1.0;
+    const double leaderAltitude = prng_uniform(side, BIRD_ALTITUDE_MIN, BIRD_ALTITUDE_MAX);
+    const double leaderSpeed = prng_uniform(side, BIRD_SPEED_MIN, BIRD_SPEED_MAX);
+    const uint64_t formationStyle = prng_next_u64(side) & 1ull;
+
+    std::vector<double> wingPhases;
+    std::vector<double> driftPhases;
+    for (int i = 0; i < expectedCount; ++i) {
+        wingPhases.push_back(prng_uniform(side, -BIRD_WING_FLAP_PHASE_JITTER, BIRD_WING_FLAP_PHASE_JITTER));
+        driftPhases.push_back(prng_uniform(side, 0.0, TwoPi));
+    }
+
+    auto flock = birds(sim);
+    REQUIRE(static_cast<int>(flock.size()) == expectedCount);
+    REQUIRE(sim.birdFlybyPrng.state == side.state);
+
+    const double spawnX = direction > 0.0 ? -50.0 : Monitor1920 + 50.0;
+    const double sinAngle = std::sin(BIRD_FLOCK_V_ANGLE_DEG * 3.14159265358979323846 / 180.0);
+    for (int i = 0; i < expectedCount; ++i) {
+        const double along = -static_cast<double>(i) * BIRD_FLOCK_FORMATION_SPACING;
+        double perpendicular = 0.0;
+        if (formationStyle == 0ull) {
+            const int armIndex = (i + 1) / 2;
+            const double sideSign = (i % 2) == 0 ? 1.0 : -1.0;
+            perpendicular = sideSign * static_cast<double>(armIndex) * BIRD_FLOCK_FORMATION_SPACING * sinAngle;
+        } else {
+            perpendicular = static_cast<double>(i) * BIRD_FLOCK_FORMATION_SPACING * sinAngle;
+        }
+
+        const Entity& e = flock[static_cast<std::size_t>(i)];
+        REQUIRE(e.x0 == Approx(spawnX + direction * along));
+        REQUIRE(e.x == Approx(e.x0));
+        REQUIRE(e.vx == Approx(direction * leaderSpeed));
+        REQUIRE(e.baseSpeed == Approx(leaderSpeed));
+        REQUIRE(e.altitudeAnchor == Approx(leaderAltitude - perpendicular));
+        REQUIRE(e.phaseX == Approx(wingPhases[static_cast<std::size_t>(i)]));
+        REQUIRE(e.phaseY == Approx(driftPhases[static_cast<std::size_t>(i)]));
+        REQUIRE(e.formationOffsetAlongFlight == Approx(along));
+        REQUIRE(e.formationOffsetPerpendicular == Approx(perpendicular));
+        REQUIRE(e.colorVariant == static_cast<uint8_t>(formationStyle));
+        REQUIRE(e.spawnTime == Approx(sim.globalTime));
+    }
+}
+
+TEST_CASE("Bird flybys are Grass scene only", "[bird][scene]") {
+    for (Scene scene : { Scene::Desert, Scene::Winter }) {
+        Sim sim = build_sim();
+        sim_set_scene(sim, scene);
+        sim.entities.clear();
+        reset_bird_schedule(sim, CANONICAL_TEST_SEED);
+        for (int i = 0; i < 8 * 3600; ++i) {
+            sim.globalTime += 1.0;
+            sim_tick_bird_flybys(sim);
+        }
+        REQUIRE(count_birds(sim) == 0);
+    }
+}
+
+TEST_CASE("Bird flyby Poisson spawns when schedule elapses", "[bird][time]") {
+    Sim sim = build_sim(0xDAD1B17Dull);
+    reset_bird_schedule(sim, 0xDAD1B17Dull);
+    int flybys = 0;
+    for (int i = 0; i < 10 * 3600; ++i) {
+        sim.globalTime += 1.0;
+        const int before = count_birds(sim);
+        sim_tick_bird_flybys(sim);
+        if (count_birds(sim) > before) {
+            ++flybys;
+            sim.entities.clear();
+        }
+    }
+
+    const double observedPerHour = static_cast<double>(flybys) / 10.0;
+    REQUIRE(observedPerHour == Approx(BIRD_FLYBY_SPAWN_RATE_PER_HOUR).epsilon(0.15));
+}
+
+TEST_CASE("Bird V formation geometry is locked", "[bird][formation]") {
+    const uint64_t seed = find_v_seed(5);
+    Sim sim = build_sim(seed);
+    reset_bird_stream_fresh(sim, seed);
+    sim_spawn_bird_flyby(sim);
+    auto flock = birds(sim);
+    REQUIRE(flock.size() >= 5);
+    REQUIRE(flock[0].colorVariant == 0);
+    REQUIRE(flock[0].formationOffsetAlongFlight == Approx(0.0));
+
+    for (std::size_t i = 1; i < flock.size(); ++i) {
+        REQUIRE(std::fabs(flock[0].formationOffsetAlongFlight)
+            < std::fabs(flock[i].formationOffsetAlongFlight));
+        REQUIRE(flock[i - 1].formationOffsetAlongFlight - flock[i].formationOffsetAlongFlight
+            == Approx(BIRD_FLOCK_FORMATION_SPACING));
+        const double expectedSign = (i % 2 == 0) ? 1.0 : -1.0;
+        REQUIRE((flock[i].formationOffsetPerpendicular > 0.0 ? 1.0 : -1.0) == expectedSign);
+    }
+}
+
+TEST_CASE("Bird wing flap scale stays in range", "[bird][wing]") {
+    for (int i = 0; i < 200; ++i) {
+        const double t = i * 0.137;
+        const double phase = -BIRD_WING_FLAP_PHASE_JITTER
+            + (2.0 * BIRD_WING_FLAP_PHASE_JITTER) * (static_cast<double>(i) / 199.0);
+        const double scale = bird_wing_scale(t, phase);
+        REQUIRE(scale >= BIRD_WING_FOLD_RATIO);
+        REQUIRE(scale <= BIRD_WING_OPEN_RATIO);
+    }
+}
+
+TEST_CASE("Bird wing phases decorrelate within a flock", "[bird][wing]") {
+    for (uint64_t i = 1; i < 10000; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i;
+        Sim sim = build_sim(seed);
+        reset_bird_stream_fresh(sim, seed);
+        sim_spawn_bird_flyby(sim);
+        auto flock = birds(sim);
+        if (flock.size() != 5) continue;
+
+        std::vector<double> distinct;
+        for (const Entity& e : flock) {
+            const double scale = bird_wing_scale(1.234, e.phaseX);
+            bool seen = false;
+            for (double existing : distinct) {
+                if (std::fabs(existing - scale) < 1e-6) { seen = true; break; }
+            }
+            if (!seen) distinct.push_back(scale);
+        }
+        if (distinct.size() >= 3) {
+            REQUIRE(distinct.size() >= 3);
+            return;
+        }
+    }
+    FAIL("no decorrelated 5-bird flock found");
+}
+
+TEST_CASE("Birds despawn past opposite boundary", "[bird][despawn]") {
+    Sim sim = build_sim();
+    sim.currentScene = Scene::Desert;
+    sim.entities.clear();
+    Entity bird{};
+    bird.kind = EntityKind::Bird;
+    bird.x = Monitor1920 + 49.0;
+    bird.y = 10.0;
+    bird.vx = 20.0;
+    bird.altitudeAnchor = BIRD_ALTITUDE_MIN;
+    bird.lifetime = -1.0;
+    sim.entities.push_back(bird);
+
+    sim_tick_entities(sim, 0.2);
+
+    REQUIRE(count_birds(sim) == 0);
+}
+
+TEST_CASE("Birds do not interact with cuts or critters", "[bird][interaction]") {
+    Sim sim = build_sim();
+    sim.entities.clear();
+    Entity bird{};
+    bird.kind = EntityKind::Bird;
+    bird.x = 500.0;
+    bird.y = sim.windowHeight - STRIP_HEIGHT - 10.0;
+    bird.vx = BIRD_SPEED_MIN;
+    bird.baseSpeed = BIRD_SPEED_MIN;
+    bird.altitudeAnchor = BIRD_ALTITUDE_MIN;
+    bird.lifetime = -1.0;
+    sim.entities.push_back(bird);
+
+    Entity sheep{};
+    sheep.kind = EntityKind::Sheep;
+    sheep.x = bird.x;
+    sheep.y = sim.windowHeight - SHEEP_BODY_HEIGHT - SHEEP_LEG_LENGTH;
+    sheep.vx = SHEEP_WALK_SPEED_MIN;
+    sheep.state = SHEEP_STATE_WALKING;
+    sheep.stateTimer = 10.0;
+    sim.entities.push_back(sheep);
+
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x = bird.x;
+    ev.y = bird.y;
+    sim_apply_click(sim, ev);
+
+    REQUIRE(sim.entities[0].kind == EntityKind::Bird);
+    REQUIRE(sim.entities[0].baseSpeed == Approx(BIRD_SPEED_MIN));
+    REQUIRE(sim.entities[1].state == SHEEP_STATE_WALKING);
+    for (const Blade& b : sim.blades) REQUIRE(b.cutAnimStart < 0.0);
+}
+
+TEST_CASE("Bird flyby Poisson inter-arrivals keep expected mean", "[bird][poisson]") {
+    const uint64_t seed = 0x510B17D00ull;
+    Sim sim = build_sim(seed);
+    reset_bird_schedule(sim, seed);
+
+    double prev = sim.globalTime;
+    double totalInterval = 0.0;
+    constexpr int Events = 100;
+    for (int i = 0; i < Events; ++i) {
+        sim.globalTime = sim.nextBirdFlybyAtTime;
+        totalInterval += sim.globalTime - prev;
+        prev = sim.globalTime;
+        sim_tick_bird_flybys(sim);
+        sim.entities.clear();
+    }
+
+    const double expectedMean = 3600.0 / BIRD_FLYBY_SPAWN_RATE_PER_HOUR;
+    REQUIRE((totalInterval / Events) == Approx(expectedMean).epsilon(0.20));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/blade_gen_tests.cpp

@@ -0,0 +1,153 @@
+// blade_gen_tests.cpp
+//
+// Snapshot + invariant tests for procedural blade generation (architecture.md §5).
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+#include "snapshot_data.h"
+
+#include <cmath>
+#include <vector>
+
+using namespace desktopgrass;
+using namespace desktopgrass::test;
+
+namespace {
+
+void requireBladeEquals(const Blade& actual, const SnapshotBlade& expected, std::size_t index) {
+    INFO("blade index = " << index);
+    REQUIRE(actual.baseX           == Approx(expected.baseX          ).margin(1e-12));
+    REQUIRE(actual.height          == Approx(expected.height         ).margin(1e-12));
+    REQUIRE(actual.thickness       == Approx(expected.thickness      ).margin(1e-12));
+    REQUIRE(actual.hue             == expected.hue);
+    REQUIRE(actual.swayPhaseOffset == Approx(expected.sway          ).margin(1e-12));
+    REQUIRE(actual.stiffness       == Approx(expected.stiffness     ).margin(1e-12));
+    REQUIRE(actual.isFlower             == expected.isFlower);
+    REQUIRE(actual.flowerHeadColorIdx   == expected.flowerHeadColorIdx);
+    REQUIRE(actual.flowerHeadRadius     == Approx(expected.flowerHeadRadius).margin(1e-12));
+    REQUIRE(actual.heightBonus          == Approx(expected.heightBonus    ).margin(1e-12));
+}
+
+} // anonymous
+
+TEST_CASE("blade generation matches the canonical snapshot", "[blade-gen][snapshot]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+
+    REQUIRE(blades.size() == CANONICAL_BLADE_COUNT);
+
+    SECTION("first 10 blades match") {
+        for (std::size_t i = 0; i < 10; ++i) {
+            requireBladeEquals(blades[i], CANONICAL_FIRST_10[i], i);
+        }
+    }
+
+    SECTION("last 10 blades match") {
+        const std::size_t start = blades.size() - 10;
+        for (std::size_t i = 0; i < 10; ++i) {
+            requireBladeEquals(blades[start + i], CANONICAL_LAST_10[i], start + i);
+        }
+    }
+}
+
+TEST_CASE("blade fields stay within spec ranges", "[blade-gen]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+
+    constexpr double TWO_PI = 6.283185307179586;
+    for (std::size_t i = 0; i < blades.size(); ++i) {
+        const Blade& b = blades[i];
+        INFO("blade index = " << i);
+        REQUIRE(b.baseX           >= 0.0);
+        REQUIRE(b.baseX           <  1920.0);
+        REQUIRE(b.height          >= BLADE_HEIGHT_MIN);
+        REQUIRE(b.height          <  BLADE_HEIGHT_MAX);
+        REQUIRE(b.thickness       >= BLADE_THICKNESS_MIN);
+        REQUIRE(b.thickness       <  BLADE_THICKNESS_MAX);
+        REQUIRE(b.hue             <  PALETTE_SIZE);
+        REQUIRE(b.swayPhaseOffset >= 0.0);
+        REQUIRE(b.swayPhaseOffset <  TWO_PI);
+        REQUIRE(b.stiffness       >= STIFFNESS_MIN);
+        REQUIRE(b.stiffness       <  STIFFNESS_MAX);
+        REQUIRE(b.cutHeight        == Approx(1.0));
+        REQUIRE(b.gustVelocity     == Approx(0.0));
+        REQUIRE(b.cutAnimStart     == Approx(-1.0));
+        REQUIRE(b.cutInitialHeight == Approx(1.0));
+        if (b.isFlower) {
+            REQUIRE(b.flowerHeadColorIdx < FLOWER_PALETTE_SIZE);
+            REQUIRE(b.flowerHeadRadius >= FLOWER_HEAD_RADIUS_MIN);
+            REQUIRE(b.flowerHeadRadius <  FLOWER_HEAD_RADIUS_MAX);
+            REQUIRE(b.heightBonus      >= FLOWER_HEIGHT_BONUS_MIN);
+            REQUIRE(b.heightBonus      <  FLOWER_HEIGHT_BONUS_MAX);
+        } else {
+            REQUIRE(b.flowerHeadColorIdx == 0);
+            REQUIRE(b.flowerHeadRadius   == Approx(0.0));
+            REQUIRE(b.heightBonus        == Approx(1.0));
+        }
+    }
+}
+
+TEST_CASE("flower count is near configured probability and ordinary blades use defaults", "[blade-gen][flowers]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    REQUIRE(blades.size() > 100);
+
+    std::size_t flowerCount = 0;
+    for (const Blade& b : blades) {
+        if (b.isFlower) {
+            ++flowerCount;
+        } else {
+            REQUIRE(b.flowerHeadColorIdx == 0);
+            REQUIRE(b.flowerHeadRadius   == Approx(0.0));
+            REQUIRE(b.heightBonus        == Approx(1.0));
+        }
+    }
+
+    const double n  = static_cast<double>(blades.size());
+    const double p  = FLOWER_PROBABILITY;
+    const double mu = n * p;
+    const double sd = std::sqrt(n * p * (1.0 - p));
+    REQUIRE(flowerCount >= static_cast<std::size_t>(std::floor(mu - 3.0 * sd)));
+    REQUIRE(flowerCount <= static_cast<std::size_t>(std::ceil(mu + 3.0 * sd)));
+}
+
+TEST_CASE("blade baseX is strictly increasing", "[blade-gen]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    REQUIRE(blades.size() > 0);
+    for (std::size_t i = 1; i < blades.size(); ++i) {
+        INFO("between " << (i-1) << " and " << i);
+        REQUIRE(blades[i].baseX > blades[i-1].baseX);
+    }
+}
+
+TEST_CASE("blade generation is deterministic across repeat runs", "[blade-gen]") {
+    std::vector<Blade> a, b;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, a);
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, b);
+    REQUIRE(a.size() == b.size());
+    for (std::size_t i = 0; i < a.size(); ++i) {
+        REQUIRE(a[i].baseX           == b[i].baseX);
+        REQUIRE(a[i].height          == b[i].height);
+        REQUIRE(a[i].thickness       == b[i].thickness);
+        REQUIRE(a[i].hue             == b[i].hue);
+        REQUIRE(a[i].swayPhaseOffset == b[i].swayPhaseOffset);
+        REQUIRE(a[i].stiffness       == b[i].stiffness);
+    }
+}
+
+TEST_CASE("density scales blade count roughly linearly", "[blade-gen]") {
+    std::vector<Blade> low, high;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 0.5, low);
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 2.0, high);
+    REQUIRE(low.size()  > 0);
+    REQUIRE(high.size() > low.size() * 3);  // 4x density ≈ 4x blades, allow slack
+}
+
+TEST_CASE("blade count near plan default at density 1.25", "[blade-gen]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.25, blades);
+    // Plan target: ~400 blades per 1920 px at the v1 default density of 1.25.
+    REQUIRE(blades.size() >= 350);
+    REQUIRE(blades.size() <= 450);
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/bunny_tests.cpp

@@ -0,0 +1,311 @@
+// bunny_tests.cpp
+//
+// §18 Bunny critter tests. Mirrors Win2D BunnyTests.cs.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cwchar>
+
+using namespace desktopgrass;
+
+namespace {
+
+constexpr double Monitor1920 = 1920.0;
+
+int count_kind(const Sim& sim, EntityKind kind) {
+    return static_cast<int>(std::count_if(sim.entities.begin(), sim.entities.end(),
+        [kind](const Entity& e) { return e.kind == kind; }));
+}
+
+Sim build_grass_sim(uint64_t seed = CANONICAL_TEST_SEED) {
+    Sim sim = sim_init(seed, Monitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Grass);
+    sim_set_critter(sim, CritterKind::Bunny);
+    return sim;
+}
+
+Entity bunny_entity(double x = 500.0, double vx = BUNNY_HOP_SPEED_MIN) {
+    Entity e{};
+    e.kind = EntityKind::Bunny;
+    e.size = BUNNY_BODY_RADIUS;
+    e.x = x;
+    e.y = STRIP_HEIGHT + HEADROOM - BUNNY_BODY_HEIGHT - BUNNY_LEG_LENGTH;
+    e.vx = vx;
+    e.rotationSpeed = std::abs(vx);
+    e.lifetime = -1.0;
+    e.state = BUNNY_STATE_HOPPING;
+    e.stateTimer = BUNNY_HOP_DURATION;
+    return e;
+}
+
+InputEvent click_event(double x, double y) {
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x = x;
+    ev.y = y;
+    ev.time = 0.0;
+    return ev;
+}
+
+int prng_count(Prng& side, int minCount, int maxCount) {
+    const double draw = prng_uniform(side, static_cast<double>(minCount), static_cast<double>(maxCount + 1));
+    int count = static_cast<int>(std::floor(draw));
+    if (count < minCount) count = minCount;
+    if (count > maxCount) count = maxCount;
+    return count;
+}
+
+void advance_sheep(Prng& side, int count) {
+    for (int i = 0; i < count; ++i) {
+        const double margin = SHEEP_BODY_RADIUS + 8.0;
+        (void)prng_uniform(side, margin, Monitor1920 - margin);
+        (void)prng_uniform(side, SHEEP_WALK_SPEED_MIN, SHEEP_WALK_SPEED_MAX);
+        (void)prng_uniform(side, 0.0, 1.0);
+        (void)prng_next_u32(side);
+        (void)prng_uniform(side, SHEEP_WALK_DURATION_MIN, SHEEP_WALK_DURATION_MAX);
+        (void)prng_index(side, static_cast<uint32_t>(sizeof(SHEEP_NAME_POOL) / sizeof(SHEEP_NAME_POOL[0])));
+    }
+}
+
+void advance_cats(Prng& side, int count) {
+    for (int i = 0; i < count; ++i) {
+        const double margin = CAT_BODY_RADIUS + 8.0;
+        (void)prng_uniform(side, margin, Monitor1920 - margin);
+        (void)prng_uniform(side, CAT_WALK_SPEED_MIN, CAT_WALK_SPEED_MAX);
+        (void)prng_uniform(side, 0.0, 1.0);
+        (void)prng_next_u32(side);
+        (void)prng_uniform(side, CAT_WALK_DURATION_MIN, CAT_WALK_DURATION_MAX);
+        (void)prng_index(side, static_cast<uint32_t>(sizeof(CAT_NAME_POOL) / sizeof(CAT_NAME_POOL[0])));
+        (void)prng_index(side, static_cast<uint32_t>(CAT_COAT_VARIANT_COUNT));
+    }
+}
+
+bool bunny_name_in_pool(const Entity& e) {
+    if (e.nameIndex >= sizeof(BUNNY_NAME_POOL) / sizeof(BUNNY_NAME_POOL[0])) return false;
+    const wchar_t* name = BUNNY_NAME_POOL[e.nameIndex];
+    for (const wchar_t* candidate : BUNNY_NAME_POOL) {
+        if (std::wcscmp(name, candidate) == 0) return true;
+    }
+    return false;
+}
+
+} // namespace
+
+TEST_CASE("Bunny constants are pinned to spec values", "[bunny][constants]") {
+    REQUIRE(BUNNY_COUNT_MIN == 1);
+    REQUIRE(BUNNY_COUNT_MAX == 2);
+    REQUIRE(BUNNY_HOP_SPEED_MIN == Approx(22.0));
+    REQUIRE(BUNNY_HOP_SPEED_MAX == Approx(38.0));
+    REQUIRE(BUNNY_BODY_RADIUS == Approx(8.0));
+    REQUIRE(BUNNY_BODY_HEIGHT == Approx(6.5));
+    REQUIRE(BUNNY_HEAD_RADIUS == Approx(4.2));
+    REQUIRE(BUNNY_EAR_HEIGHT == Approx(9.0));
+    REQUIRE(BUNNY_EAR_WIDTH == Approx(2.2));
+    REQUIRE(BUNNY_EAR_SPACING == Approx(3.0));
+    REQUIRE(BUNNY_LEG_LENGTH == Approx(4.0));
+    REQUIRE(BUNNY_TAIL_RADIUS == Approx(2.4));
+    REQUIRE(BUNNY_BODY_COLOR == 0xFF8A6A4Au);
+    REQUIRE(BUNNY_BELLY_COLOR == 0xFFC4A98Du);
+    REQUIRE(BUNNY_EAR_COLOR == 0xFF8A6A4Au);
+    REQUIRE(BUNNY_EAR_INNER_COLOR == 0xFFD9A0A0u);
+    REQUIRE(BUNNY_TAIL_COLOR == 0xFFF7F4EBu);
+    REQUIRE(BUNNY_EYE_COLOR == 0xFF1A1208u);
+    REQUIRE(BUNNY_NOSE_COLOR == 0xFF8A4040u);
+    REQUIRE(BUNNY_STATE_HOPPING == 0);
+    REQUIRE(BUNNY_STATE_GRAZING == 1);
+    REQUIRE(BUNNY_STATE_IDLE == 2);
+    REQUIRE(BUNNY_STATE_SLEEPING == 3);
+    REQUIRE(BUNNY_STATE_STARTLED == 4);
+    REQUIRE(BUNNY_HOP_DURATION == Approx(0.40));
+    REQUIRE(BUNNY_HOP_HEIGHT == Approx(8.0));
+    REQUIRE(BUNNY_HOP_GAP_MIN == Approx(0.05));
+    REQUIRE(BUNNY_HOP_GAP_MAX == Approx(0.20));
+    REQUIRE(BUNNY_GRAZE_DURATION_MIN == Approx(2.5));
+    REQUIRE(BUNNY_GRAZE_DURATION_MAX == Approx(4.5));
+    REQUIRE(BUNNY_IDLE_DURATION_MIN == Approx(2.0));
+    REQUIRE(BUNNY_IDLE_DURATION_MAX == Approx(4.0));
+    REQUIRE(BUNNY_SLEEP_DURATION_MIN == Approx(6.0));
+    REQUIRE(BUNNY_SLEEP_DURATION_MAX == Approx(12.0));
+    REQUIRE(BUNNY_GRAZE_PROBABILITY == Approx(0.55));
+    REQUIRE(BUNNY_IDLE_PROBABILITY == Approx(0.30));
+    REQUIRE(BUNNY_SLEEP_PROB == Approx(0.05));
+    REQUIRE(BUNNY_STARTLE_RADIUS == Approx(90.0));
+    REQUIRE(BUNNY_STARTLE_BOOST == Approx(2.0));
+    REQUIRE(BUNNY_STARTLE_HOP_HEIGHT == Approx(14.0));
+    REQUIRE(BUNNY_STARTLE_DURATION == Approx(3.0));
+    REQUIRE(BUNNY_NOSE_TWITCH_FREQ == Approx(6.0));
+    REQUIRE(BUNNY_NOSE_TWITCH_AMP == Approx(0.5));
+    REQUIRE(BUNNY_EAR_WIGGLE_FREQ == Approx(1.2));
+    REQUIRE(BUNNY_EAR_WIGGLE_AMP == Approx(0.20));
+    REQUIRE(BUNNY_ZZZ_CYCLE_SEC == Approx(SHEEP_ZZZ_CYCLE_SEC));
+    REQUIRE(BUNNY_ZZZ_RISE == Approx(SHEEP_ZZZ_RISE * 0.7));
+    REQUIRE(BUNNY_ZZZ_SIZE_START == Approx(SHEEP_ZZZ_SIZE_START * 0.7));
+    REQUIRE(BUNNY_ZZZ_SIZE_END == Approx(SHEEP_ZZZ_SIZE_END * 0.7));
+    REQUIRE(sizeof(BUNNY_NAME_POOL) / sizeof(BUNNY_NAME_POOL[0]) == 12);
+    REQUIRE(std::wcscmp(BUNNY_NAME_POOL[0], L"Clover") == 0);
+    REQUIRE(std::wcscmp(BUNNY_NAME_POOL[11], L"Snowdrop") == 0);
+}
+
+TEST_CASE("Grass generation produces bunny count in range", "[bunny][gen]") {
+    for (uint64_t i = 0; i < 128; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i * 0x9E3779B97F4A7C15ull;
+        Sim sim = build_grass_sim(seed);
+        const int bunnies = count_kind(sim, EntityKind::Bunny);
+        REQUIRE(bunnies >= BUNNY_COUNT_MIN);
+        REQUIRE(bunnies <= BUNNY_COUNT_MAX);
+    }
+}
+
+TEST_CASE("Bunnies are Grass scene only", "[bunny][scene]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, Monitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE(count_kind(sim, EntityKind::Bunny) == 0);
+    sim_set_scene(sim, Scene::Winter);
+    REQUIRE(count_kind(sim, EntityKind::Bunny) == 0);
+    sim_set_critter(sim, CritterKind::Bunny);
+    REQUIRE(count_kind(sim, EntityKind::Bunny) == 0);
+}
+
+TEST_CASE("Generated bunnies have speed range", "[bunny][gen]") {
+    Sim sim = build_grass_sim();
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Bunny) continue;
+        REQUIRE(std::abs(e.vx) >= BUNNY_HOP_SPEED_MIN);
+        REQUIRE(std::abs(e.vx) < BUNNY_HOP_SPEED_MAX);
+        REQUIRE(e.rotationSpeed == Approx(std::abs(e.vx)));
+    }
+}
+
+TEST_CASE("Generated bunnies have names in pool", "[bunny][gen]") {
+    Sim sim = build_grass_sim();
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Bunny) continue;
+        REQUIRE(bunny_name_in_pool(e));
+    }
+}
+
+TEST_CASE("Bunny PRNG draw order follows sheep and cats", "[bunny][prng]") {
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+
+    Sim sim = build_grass_sim();
+
+    const int sheepCount = prng_count(side, SHEEP_COUNT_MIN, SHEEP_COUNT_MAX);
+    advance_sheep(side, sheepCount);
+    const int catCount = prng_count(side, CAT_COUNT_MIN, CAT_COUNT_MAX);
+    advance_cats(side, catCount);
+    const int bunnyCount = prng_count(side, BUNNY_COUNT_MIN, BUNNY_COUNT_MAX);
+    REQUIRE(count_kind(sim, EntityKind::Bunny) == bunnyCount);
+
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Bunny) continue;
+        const double margin = BUNNY_BODY_RADIUS + 8.0;
+        const double xFrac = prng_uniform(side, 0.0, 1.0);
+        const double expectedX = margin + xFrac * (Monitor1920 - 2.0 * margin);
+        const uint64_t vxSign = prng_next_u64(side) & 1ull;
+        const double expectedDir = vxSign != 0ull ? 1.0 : -1.0;
+        const double expectedSpeed = prng_uniform(side, BUNNY_HOP_SPEED_MIN, BUNNY_HOP_SPEED_MAX);
+        const uint8_t expectedName = static_cast<uint8_t>(prng_index(side,
+            static_cast<uint32_t>(sizeof(BUNNY_NAME_POOL) / sizeof(BUNNY_NAME_POOL[0]))));
+        REQUIRE(e.x == Approx(expectedX));
+        REQUIRE(e.vx == Approx(expectedDir * expectedSpeed));
+        REQUIRE(e.nameIndex == expectedName);
+        ++seen;
+    }
+    REQUIRE(seen == bunnyCount);
+}
+
+TEST_CASE("Bunny edge bounce flips direction", "[bunny][motion]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, Monitor1920, DEFAULT_DENSITY);
+    sim.currentScene = Scene::Desert;
+    sim.entities.clear();
+    Entity e = bunny_entity(Monitor1920 - (BUNNY_BODY_RADIUS + 2.0) + 0.1, BUNNY_HOP_SPEED_MIN);
+    e.stateTimer = 10.0;
+    sim.entities.push_back(e);
+
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(sim.entities.front().vx < 0.0);
+}
+
+TEST_CASE("Bunny startle radius hops away and outside click does nothing", "[bunny][click]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, Monitor1920, DEFAULT_DENSITY);
+    sim.entities.clear();
+    Entity e = bunny_entity(500.0, -BUNNY_HOP_SPEED_MIN);
+    e.state = BUNNY_STATE_IDLE;
+    e.stateTimer = 3.0;
+    sim.entities.push_back(e);
+
+    sim_apply_click(sim, click_event(500.0 - 20.0, e.y));
+    REQUIRE(sim.entities.front().state == BUNNY_STATE_STARTLED);
+    REQUIRE(sim.entities.front().vx > 0.0);
+    REQUIRE(sim.entities.front().stateTimer == Approx(BUNNY_STARTLE_DURATION));
+
+    Entity after = sim.entities.front();
+    const double vxBefore = after.vx;
+    const uint8_t stateBefore = after.state;
+    sim_apply_click(sim, click_event(after.x + BUNNY_STARTLE_RADIUS + 10.0, after.y));
+    REQUIRE(sim.entities.front().state == stateBefore);
+    REQUIRE(sim.entities.front().vx == Approx(vxBefore));
+}
+
+TEST_CASE("Bunny wakes from sleep on startle", "[bunny][click]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, Monitor1920, DEFAULT_DENSITY);
+    sim.entities.clear();
+    Entity e = bunny_entity(500.0, BUNNY_HOP_SPEED_MIN);
+    e.state = BUNNY_STATE_SLEEPING;
+    e.stateTimer = 10.0;
+    sim.entities.push_back(e);
+
+    sim_apply_click(sim, click_event(e.x + 10.0, e.y));
+
+    REQUIRE(sim.entities.front().state == BUNNY_STATE_STARTLED);
+    REQUIRE(sim.entities.front().state != BUNNY_STATE_SLEEPING);
+    REQUIRE(sim.entities.front().vx < 0.0);
+}
+
+TEST_CASE("Bunny hop arc is bounded", "[bunny][motion]") {
+    REQUIRE(bunny_hop_y_offset(0.0, false) == Approx(0.0));
+    REQUIRE(bunny_hop_y_offset(BUNNY_HOP_DURATION, false) == Approx(0.0).margin(1e-12));
+    const double peak = bunny_hop_y_offset(BUNNY_HOP_DURATION * 0.5, false);
+    REQUIRE(peak > 0.0);
+    REQUIRE(peak <= BUNNY_HOP_HEIGHT);
+}
+
+TEST_CASE("Bunny state transition probabilities are stable", "[bunny][state]") {
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+    constexpr int N = 10000;
+    int graze = 0;
+    int idle = 0;
+    int sleep = 0;
+    for (int i = 0; i < N; ++i) {
+        const uint8_t state = bunny_choose_rest_state(p);
+        if (state == BUNNY_STATE_GRAZING) ++graze;
+        else if (state == BUNNY_STATE_IDLE) ++idle;
+        else if (state == BUNNY_STATE_SLEEPING) ++sleep;
+    }
+
+    const double sleepProb = BUNNY_SLEEP_PROB;
+    const double activeWeight = BUNNY_GRAZE_PROBABILITY + BUNNY_IDLE_PROBABILITY;
+    const double expectedGraze = (1.0 - sleepProb) * BUNNY_GRAZE_PROBABILITY / activeWeight;
+    const double expectedIdle = (1.0 - sleepProb) * BUNNY_IDLE_PROBABILITY / activeWeight;
+    REQUIRE(static_cast<double>(sleep) / N == Approx(sleepProb).margin(0.02));
+    REQUIRE(static_cast<double>(graze) / N == Approx(expectedGraze).margin(0.02));
+    REQUIRE(static_cast<double>(idle) / N == Approx(expectedIdle).margin(0.02));
+}
+
+TEST_CASE("Bunny sleep probability is stable", "[bunny][state]") {
+    constexpr int N = 20000;
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED ^ 0x1234ull);
+    int sleep = 0;
+    for (int i = 0; i < N; ++i) {
+        if (bunny_choose_rest_state(p) == BUNNY_STATE_SLEEPING) ++sleep;
+    }
+    REQUIRE(static_cast<double>(sleep) / N == Approx(BUNNY_SLEEP_PROB).margin(0.02));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/butterfly_tests.cpp

@@ -0,0 +1,180 @@
+// butterfly_tests.cpp - §17.6 ambient Butterfly tests.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+#include <cmath>
+
+using namespace desktopgrass;
+
+namespace {
+constexpr double Monitor1920 = 1920.0;
+
+int count_kind(const Sim& sim, EntityKind kind) {
+    return static_cast<int>(std::count_if(sim.entities.begin(), sim.entities.end(),
+        [kind](const Entity& e) { return e.kind == kind; }));
+}
+
+Sim build_grass_sim(uint64_t seed = CANONICAL_TEST_SEED) {
+    Sim sim = sim_init(seed, Monitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Grass);
+    return sim;
+}
+
+int prng_count(Prng& side, int minCount, int maxCount) {
+    const double draw = prng_uniform(side, static_cast<double>(minCount), static_cast<double>(maxCount + 1));
+    int count = static_cast<int>(std::floor(draw));
+    if (count < minCount) count = minCount;
+    if (count > maxCount) count = maxCount;
+    return count;
+}
+
+const Entity* first_butterfly(const Sim& sim) {
+    for (const Entity& e : sim.entities) if (e.kind == EntityKind::Butterfly) return &e;
+    return nullptr;
+}
+} // namespace
+
+TEST_CASE("Butterfly constants are pinned to spec values", "[butterfly][constants]") {
+    REQUIRE(BUTTERFLY_COUNT_MIN == 2);
+    REQUIRE(BUTTERFLY_COUNT_MAX == 4);
+    REQUIRE(BUTTERFLY_SPEED_MIN == Approx(18.0));
+    REQUIRE(BUTTERFLY_SPEED_MAX == Approx(32.0));
+    REQUIRE(BUTTERFLY_BODY_LENGTH == Approx(2.4));
+    REQUIRE(BUTTERFLY_WING_RADIUS == Approx(3.5));
+    REQUIRE(BUTTERFLY_WING_OFFSET == Approx(2.2));
+    REQUIRE(BUTTERFLY_FLUTTER_FREQ == Approx(16.0));
+    REQUIRE(BUTTERFLY_FLUTTER_MIN_SCALE == Approx(0.20));
+    REQUIRE(BUTTERFLY_MEANDER_FREQ_Y == Approx(0.8));
+    REQUIRE(BUTTERFLY_MEANDER_AMP_Y == Approx(16.0));
+    REQUIRE(BUTTERFLY_MEANDER_FREQ_X == Approx(0.5));
+    REQUIRE(BUTTERFLY_MEANDER_AMP_X == Approx(0.4));
+    REQUIRE(BUTTERFLY_ALTITUDE_MIN == Approx(18.0));
+    REQUIRE(BUTTERFLY_ALTITUDE_MAX == Approx(70.0));
+    REQUIRE(BUTTERFLY_BODY_COLOR == 0xFF2A2018u);
+    REQUIRE(BUTTERFLY_COLOR_COUNT == 5);
+    REQUIRE(BUTTERFLY_PRNG_SALT == 0xB07DEF1E0001ull);
+}
+
+TEST_CASE("Grass generation produces butterfly count in range", "[butterfly][gen]") {
+    for (uint64_t i = 0; i < 128; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i * 0x9E3779B97F4A7C15ull;
+        Sim sim = build_grass_sim(seed);
+        REQUIRE(count_kind(sim, EntityKind::Butterfly) >= BUTTERFLY_COUNT_MIN);
+        REQUIRE(count_kind(sim, EntityKind::Butterfly) <= BUTTERFLY_COUNT_MAX);
+    }
+}
+
+TEST_CASE("Butterflies are Grass scene only", "[butterfly][scene]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, Monitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE(count_kind(sim, EntityKind::Butterfly) == 0);
+    sim_set_scene(sim, Scene::Winter);
+    REQUIRE(count_kind(sim, EntityKind::Butterfly) == 0);
+}
+
+TEST_CASE("Generated butterflies have speed altitude and color ranges", "[butterfly][gen]") {
+    Sim sim = build_grass_sim();
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Butterfly) continue;
+        REQUIRE(e.baseSpeed >= BUTTERFLY_SPEED_MIN);
+        REQUIRE(e.baseSpeed <  BUTTERFLY_SPEED_MAX);
+        REQUIRE(e.altitudeAnchor >= BUTTERFLY_ALTITUDE_MIN);
+        REQUIRE(e.altitudeAnchor <  BUTTERFLY_ALTITUDE_MAX);
+        REQUIRE(e.colorVariant < BUTTERFLY_COLOR_COUNT);
+    }
+}
+
+TEST_CASE("Butterfly PRNG draw order matches side stream", "[butterfly][prng]") {
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ BUTTERFLY_PRNG_SALT);
+    Sim sim = build_grass_sim();
+
+    const int expectedCount = prng_count(side, BUTTERFLY_COUNT_MIN, BUTTERFLY_COUNT_MAX);
+    REQUIRE(count_kind(sim, EntityKind::Butterfly) == expectedCount);
+
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Butterfly) continue;
+        const double xFrac = prng_uniform(side, 0.0, 1.0);
+        const double yFrac = prng_uniform(side, 0.0, 1.0);
+        const uint64_t vxSign = prng_next_u64(side) & 1ull;
+        const double expectedDir = vxSign != 0ull ? 1.0 : -1.0;
+        const double expectedSpeed = prng_uniform(side, BUTTERFLY_SPEED_MIN, BUTTERFLY_SPEED_MAX);
+        const uint8_t expectedColor = static_cast<uint8_t>(prng_index(side, static_cast<uint32_t>(BUTTERFLY_COLOR_COUNT)));
+        const double expectedPhaseY = prng_uniform(side, 0.0, 2.0 * 3.14159265358979323846);
+        const double expectedPhaseX = prng_uniform(side, 0.0, 2.0 * 3.14159265358979323846);
+        const double expectedAltitude = BUTTERFLY_ALTITUDE_MIN + yFrac * (BUTTERFLY_ALTITUDE_MAX - BUTTERFLY_ALTITUDE_MIN);
+        const double expectedVx = expectedDir * expectedSpeed * (1.0 + BUTTERFLY_MEANDER_AMP_X * std::sin(expectedPhaseX));
+
+        REQUIRE(e.x == Approx(xFrac * Monitor1920));
+        REQUIRE(e.altitudeAnchor == Approx(expectedAltitude));
+        REQUIRE(e.baseSpeed == Approx(expectedSpeed));
+        REQUIRE(e.vx == Approx(expectedVx));
+        REQUIRE(e.colorVariant == expectedColor);
+        REQUIRE(e.phaseY == Approx(expectedPhaseY));
+        REQUIRE(e.phaseX == Approx(expectedPhaseX));
+        ++seen;
+    }
+    REQUIRE(seen == expectedCount);
+}
+
+TEST_CASE("Butterfly edge wrap preserves altitude anchor", "[butterfly][motion]") {
+    Sim sim = build_grass_sim();
+    auto it = std::find_if(sim.entities.begin(), sim.entities.end(), [](const Entity& e) { return e.kind == EntityKind::Butterfly; });
+    REQUIRE(it != sim.entities.end());
+    const double margin = BUTTERFLY_WING_OFFSET + BUTTERFLY_WING_RADIUS;
+    it->x = Monitor1920 + margin + 1.0;
+    it->vx = std::abs(it->vx);
+    const double altitude = it->altitudeAnchor;
+    sim.currentScene = Scene::Desert;
+
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(it->x == Approx(-margin));
+    REQUIRE(it->altitudeAnchor == Approx(altitude));
+}
+
+TEST_CASE("Butterflies do not interact with cuts or pets", "[butterfly][interaction]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, Monitor1920, DEFAULT_DENSITY);
+    sim.entities.clear();
+    Entity butterfly{};
+    butterfly.kind = EntityKind::Butterfly;
+    butterfly.x = 500.0;
+    butterfly.y = sim.windowHeight - STRIP_HEIGHT - 5.0;
+    butterfly.vx = BUTTERFLY_SPEED_MIN;
+    butterfly.baseSpeed = BUTTERFLY_SPEED_MIN;
+    butterfly.altitudeAnchor = BUTTERFLY_ALTITUDE_MIN;
+    butterfly.lifetime = -1.0;
+    sim.entities.push_back(butterfly);
+    Entity sheep{};
+    sheep.kind = EntityKind::Sheep;
+    sheep.x = butterfly.x;
+    sheep.y = sim.windowHeight - SHEEP_BODY_HEIGHT - SHEEP_LEG_LENGTH;
+    sheep.vx = SHEEP_WALK_SPEED_MIN;
+    sheep.state = SHEEP_STATE_WALKING;
+    sheep.stateTimer = 10.0;
+    sim.entities.push_back(sheep);
+
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x = butterfly.x;
+    ev.y = butterfly.y;
+    sim_apply_click(sim, ev);
+
+    REQUIRE(sim.entities[0].kind == EntityKind::Butterfly);
+    REQUIRE(sim.entities[0].baseSpeed == Approx(BUTTERFLY_SPEED_MIN));
+    REQUIRE(sim.entities[1].state == SHEEP_STATE_WALKING);
+    for (const Blade& b : sim.blades) REQUIRE(b.cutAnimStart < 0.0);
+}
+
+TEST_CASE("Butterfly wing scale stays within flutter bounds", "[butterfly][render]") {
+    for (int i = 0; i < 200; ++i) {
+        const double t = i * 0.05;
+        const double scale = butterfly_wing_scale(t, 1.3);
+        REQUIRE(scale >= BUTTERFLY_FLUTTER_MIN_SCALE);
+        REQUIRE(scale <= 1.0);
+    }
+}
+

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/cat_coat_tests.cpp

@@ -0,0 +1,158 @@
+// cat_coat_tests.cpp
+//
+// §17 Cat coat palette and deterministic coat variant tests. Mirrors Win2D CatCoatTests.cs.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <cmath>
+#include <cstdint>
+
+using namespace desktopgrass;
+
+namespace {
+
+int count_kind(const Sim& sim, EntityKind kind) {
+    int n = 0;
+    for (const Entity& e : sim.entities) if (e.kind == kind) ++n;
+    return n;
+}
+
+constexpr CatCoatPalette EXPECTED_CAT_COATS[CAT_COAT_VARIANT_COUNT] = {
+    { 0xFF6B6259u, 0xFF3D3733u, 0xFF6B6259u, 0xFF3D3733u, 0xFF1A1614u },
+    { 0xFFD89A6Fu, 0xFFA56B40u, 0xFFD89A6Fu, 0xFFA56B40u, 0xFF2B1A0Eu },
+    { 0xFF2A2522u, 0xFF140F0Cu, 0xFF2A2522u, 0xFF140F0Cu, 0xFFD9B85Bu },
+    { 0xFFEDE9E1u, 0xFFBDB7ABu, 0xFFEDE9E1u, 0xFFBDB7ABu, 0xFF1F1817u },
+    { 0xFF7A5F3Cu, 0xFF4E3F26u, 0xFF7A5F3Cu, 0xFF4E3F26u, 0xFF1A1108u },
+    { 0xFFC9B898u, 0xFF8E7F6Bu, 0xFFC9B898u, 0xFF8E7F6Bu, 0xFF2E251Du },
+};
+
+uint8_t next_cat_coat_after_prefix(Prng& side) {
+    (void)prng_uniform(side, CAT_BODY_RADIUS + 8.0, 1920.0 - (CAT_BODY_RADIUS + 8.0));
+    (void)prng_uniform(side, CAT_WALK_SPEED_MIN, CAT_WALK_SPEED_MAX);
+    (void)prng_uniform(side, 0.0, 1.0);
+    (void)prng_next_u32(side);
+    (void)prng_uniform(side, CAT_WALK_DURATION_MIN, CAT_WALK_DURATION_MAX);
+    (void)prng_index(side, static_cast<uint32_t>(sizeof(CAT_NAME_POOL) / sizeof(CAT_NAME_POOL[0])));
+    return static_cast<uint8_t>(prng_index(side, static_cast<uint32_t>(CAT_COAT_VARIANT_COUNT)));
+}
+
+} // namespace
+
+TEST_CASE("Cat coat variant count is pinned", "[cat][coat][constants]") {
+    REQUIRE(CAT_COAT_VARIANT_COUNT == 6);
+}
+
+TEST_CASE("Cat coat palette zero matches backward-compatible aliases", "[cat][coat][constants]") {
+    REQUIRE(CAT_COAT_PALETTES[0].body == CAT_BODY_COLOR);
+    REQUIRE(CAT_COAT_PALETTES[0].leg  == CAT_LEG_COLOR);
+    REQUIRE(CAT_COAT_PALETTES[0].face == CAT_FACE_COLOR);
+    REQUIRE(CAT_COAT_PALETTES[0].ear  == CAT_EAR_COLOR);
+    REQUIRE(CAT_COAT_PALETTES[0].ink  == CAT_INK_COLOR);
+}
+
+TEST_CASE("All cat coat palettes are pinned", "[cat][coat][constants]") {
+    for (int i = 0; i < CAT_COAT_VARIANT_COUNT; ++i) {
+        CAPTURE(i);
+        REQUIRE(CAT_COAT_PALETTES[i].body == EXPECTED_CAT_COATS[i].body);
+        REQUIRE(CAT_COAT_PALETTES[i].leg  == EXPECTED_CAT_COATS[i].leg);
+        REQUIRE(CAT_COAT_PALETTES[i].face == EXPECTED_CAT_COATS[i].face);
+        REQUIRE(CAT_COAT_PALETTES[i].ear  == EXPECTED_CAT_COATS[i].ear);
+        REQUIRE(CAT_COAT_PALETTES[i].ink  == EXPECTED_CAT_COATS[i].ink);
+    }
+}
+
+TEST_CASE("Cat coat body colors are distinct", "[cat][coat][constants]") {
+    for (int i = 0; i < CAT_COAT_VARIANT_COUNT; ++i) {
+        for (int j = i + 1; j < CAT_COAT_VARIANT_COUNT; ++j) {
+            CAPTURE(i);
+            CAPTURE(j);
+            REQUIRE(CAT_COAT_PALETTES[i].body != CAT_COAT_PALETTES[j].body);
+        }
+    }
+}
+
+TEST_CASE("Canonical cat flock pins deterministic coat variants", "[cat][coat][gen]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+
+    const uint8_t expectedCoats[] = { 1 };
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Cat) continue;
+        REQUIRE(seen < static_cast<int>(sizeof(expectedCoats) / sizeof(expectedCoats[0])));
+        REQUIRE(e.coatVariantIndex == expectedCoats[seen]);
+        ++seen;
+    }
+    REQUIRE(seen == static_cast<int>(sizeof(expectedCoats) / sizeof(expectedCoats[0])));
+}
+
+TEST_CASE("Cat coat PRNG draw follows nameIndex", "[cat][coat][prng]") {
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+
+    const double countDraw = prng_uniform(side, CAT_COUNT_MIN, CAT_COUNT_MAX + 1);
+    int expectedCount = static_cast<int>(std::floor(countDraw));
+    if (expectedCount < CAT_COUNT_MIN) expectedCount = CAT_COUNT_MIN;
+    if (expectedCount > CAT_COUNT_MAX) expectedCount = CAT_COUNT_MAX;
+    REQUIRE(count_kind(sim, EntityKind::Cat) == expectedCount);
+
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Cat) continue;
+        const uint8_t expectedCoat = next_cat_coat_after_prefix(side);
+        REQUIRE(e.coatVariantIndex == expectedCoat);
+        ++seen;
+    }
+    REQUIRE(seen == expectedCount);
+}
+
+TEST_CASE("Generated cat coats always stay within palette range", "[cat][coat][gen]") {
+    for (uint64_t i = 0; i < 128; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i * 0x9E3779B97F4A7C15ull;
+        Sim sim = sim_init(seed, 1920.0, DEFAULT_DENSITY);
+        sim_set_critter(sim, CritterKind::Cat);
+
+        int seen = 0;
+        for (const Entity& e : sim.entities) {
+            if (e.kind != EntityKind::Cat) continue;
+            REQUIRE(e.coatVariantIndex < CAT_COAT_VARIANT_COUNT);
+            ++seen;
+        }
+        REQUIRE(seen >= CAT_COUNT_MIN);
+        REQUIRE(seen <= CAT_COUNT_MAX);
+    }
+}
+
+TEST_CASE("Sheep keep default coat variant zero", "[cat][coat][sheep]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+
+    REQUIRE(count_kind(sim, EntityKind::Sheep) >= SHEEP_COUNT_MIN);
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Sheep) continue;
+        REQUIRE(e.coatVariantIndex == 0);
+    }
+}
+
+TEST_CASE("Fixed cat count coat PRNG skips only the count draw", "[cat][coat][count][prng]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+    sim_set_critter_count(sim, 3);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 3);
+
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Cat) continue;
+        const uint8_t expectedCoat = next_cat_coat_after_prefix(side);
+        REQUIRE(e.coatVariantIndex == expectedCoat);
+        ++seen;
+    }
+    REQUIRE(seen == 3);
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/cat_tests.cpp

@@ -0,0 +1,332 @@
+// cat_tests.cpp
+//
+// §17 Cat critter tests. Mirrors Win2D CatTests.cs.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+#include <cmath>
+
+using namespace desktopgrass;
+
+namespace {
+
+int count_kind(const Sim& sim, EntityKind kind) {
+    return static_cast<int>(std::count_if(sim.entities.begin(), sim.entities.end(),
+        [kind](const Entity& e) { return e.kind == kind; }));
+}
+
+Entity* first_kind(Sim& sim, EntityKind kind) {
+    for (Entity& e : sim.entities) if (e.kind == kind) return &e;
+    return nullptr;
+}
+
+const Entity* first_kind(const Sim& sim, EntityKind kind) {
+    for (const Entity& e : sim.entities) if (e.kind == kind) return &e;
+    return nullptr;
+}
+
+void keep_first_cat_only(Sim& sim) {
+    Entity* cat = first_kind(sim, EntityKind::Cat);
+    REQUIRE(cat != nullptr);
+    const Entity copy = *cat;
+    sim.entities.clear();
+    sim.entities.push_back(copy);
+}
+
+InputEvent click_event(double x, double y) {
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x = x;
+    ev.y = y;
+    ev.time = 0.0;
+    return ev;
+}
+
+} // namespace
+
+TEST_CASE("CritterKind::Cat and CRITTER_COUNT are pinned", "[cat][enum]") {
+    REQUIRE(static_cast<int>(CritterKind::None)  == 0);
+    REQUIRE(static_cast<int>(CritterKind::Sheep) == 1);
+    REQUIRE(static_cast<int>(CritterKind::Cat)   == 2);
+    REQUIRE(static_cast<int>(CritterKind::Bunny) == 3);
+    REQUIRE(CRITTER_COUNT == 4);
+    REQUIRE(CRITTER_DEFAULT == CritterKind::None);
+}
+
+TEST_CASE("EntityKind::Cat is pinned", "[cat][enum]") {
+    REQUIRE(static_cast<int>(EntityKind::None)       == 0);
+    REQUIRE(static_cast<int>(EntityKind::Tumbleweed) == 1);
+    REQUIRE(static_cast<int>(EntityKind::Snowflake)  == 2);
+    REQUIRE(static_cast<int>(EntityKind::Sheep)      == 3);
+    REQUIRE(static_cast<int>(EntityKind::Cat)        == 4);
+}
+
+TEST_CASE("Cat constants are pinned to spec values", "[cat][constants]") {
+    REQUIRE(CAT_COUNT_MIN == 1);
+    REQUIRE(CAT_COUNT_MAX == 2);
+    REQUIRE(CAT_WALK_SPEED_MIN == Approx(10.0));
+    REQUIRE(CAT_WALK_SPEED_MAX == Approx(22.0));
+    REQUIRE(CAT_POUNCE_SPEED   == Approx(60.0));
+
+    REQUIRE(CAT_BODY_RADIUS    == Approx(11.0));
+    REQUIRE(CAT_BODY_HEIGHT    == Approx(7.0));
+    REQUIRE(CAT_HEAD_RADIUS    == Approx(4.5));
+    REQUIRE(CAT_LEG_LENGTH     == Approx(5.0));
+    REQUIRE(CAT_TAIL_LENGTH    == Approx(13.0));
+    REQUIRE(CAT_TAIL_THICKNESS == Approx(1.6));
+    REQUIRE(CAT_EAR_HEIGHT     == Approx(4.5));
+
+    REQUIRE(CAT_BODY_COLOR == 0xFF6B6259u);
+    REQUIRE(CAT_LEG_COLOR  == 0xFF3D3733u);
+    REQUIRE(CAT_FACE_COLOR == 0xFF6B6259u);
+    REQUIRE(CAT_EAR_COLOR  == 0xFF3D3733u);
+    REQUIRE(CAT_INK_COLOR  == 0xFF1A1614u);
+
+    REQUIRE(CAT_WALK_PERIOD    == Approx(0.50));
+    REQUIRE(CAT_LEG_CYCLE_AMP  == Approx(1.6));
+    REQUIRE(CAT_HEAD_BOB_AMP   == Approx(0.4));
+    REQUIRE(CAT_TAIL_SWAY_FREQ == Approx(1.2));
+    REQUIRE(CAT_TAIL_SWAY_AMP  == Approx(0.35));
+
+    REQUIRE(CAT_STATE_WALKING  == SHEEP_STATE_WALKING);
+    REQUIRE(CAT_STATE_IDLE     == SHEEP_STATE_IDLE);
+    REQUIRE(CAT_STATE_SLEEPING == SHEEP_STATE_SLEEPING);
+    REQUIRE(CAT_STATE_POUNCING == SHEEP_STATE_HOPPING);
+
+    REQUIRE(CAT_WALK_DURATION_MIN  == Approx(6.0));
+    REQUIRE(CAT_WALK_DURATION_MAX  == Approx(10.0));
+    REQUIRE(CAT_IDLE_DURATION_MIN  == Approx(4.0));
+    REQUIRE(CAT_IDLE_DURATION_MAX  == Approx(8.0));
+    REQUIRE(CAT_SLEEP_DURATION_MIN == Approx(20.0));
+    REQUIRE(CAT_SLEEP_DURATION_MAX == Approx(40.0));
+    REQUIRE(CAT_POUNCE_DURATION    == Approx(0.45));
+
+    REQUIRE(CAT_IDLE_PROBABILITY == Approx(0.65));
+    REQUIRE(CAT_SLEEP_PROBABILITY == Approx(0.30));
+    REQUIRE(CAT_SLEEP_FROM_IDLE_PROB == Approx(0.50));
+
+    REQUIRE(CAT_POUNCE_RADIUS == Approx(80.0));
+    REQUIRE(CAT_POUNCE_HEIGHT == Approx(9.0));
+    REQUIRE(CAT_CURIOUS_RADIUS == Approx(100.0));
+    REQUIRE(CAT_CURIOUS_HEAD_TURN_MAX == Approx(0.7));
+}
+
+TEST_CASE("sim_init defaults to None and does not generate cats until selected", "[cat][init]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    REQUIRE(sim.currentCritter == CritterKind::None);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 0);
+
+    sim_set_critter(sim, CritterKind::Cat);
+    REQUIRE(count_kind(sim, EntityKind::Cat) >= CAT_COUNT_MIN);
+}
+
+TEST_CASE("sim_set_critter(Cat) produces deterministic cats", "[cat][gen]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+
+    REQUIRE(sim.currentCritter == CritterKind::Cat);
+    const int k = count_kind(sim, EntityKind::Cat);
+    REQUIRE(k >= CAT_COUNT_MIN);
+    REQUIRE(k <= CAT_COUNT_MAX);
+
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Cat) continue;
+        REQUIRE(e.state == CAT_STATE_WALKING);
+        REQUIRE(e.stateTimer >= CAT_WALK_DURATION_MIN);
+        REQUIRE(e.stateTimer <  CAT_WALK_DURATION_MAX);
+        REQUIRE(std::fabs(e.vx) >= CAT_WALK_SPEED_MIN);
+        REQUIRE(std::fabs(e.vx) <  CAT_WALK_SPEED_MAX);
+        const double margin = e.size + 8.0;
+        REQUIRE(e.x >= margin);
+        REQUIRE(e.x <= sim.monitorWidth - margin);
+        REQUIRE(e.y == Approx(sim.windowHeight - CAT_BODY_HEIGHT - CAT_LEG_LENGTH));
+        REQUIRE(e.size == Approx(CAT_BODY_RADIUS));
+        REQUIRE(e.lifetime < 0.0);
+        REQUIRE(e.nameIndex < sizeof(CAT_NAME_POOL) / sizeof(CAT_NAME_POOL[0]));
+        REQUIRE(e.coatVariantIndex < CAT_COAT_VARIANT_COUNT);
+    }
+}
+
+TEST_CASE("Cat PRNG draw order matches a side stream", "[cat][prng]") {
+    // count, then per-cat: x, speed, dir-coin, seed, stateTimer, nameIndex, coatVariantIndex
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+
+    const double countDraw = prng_uniform(side, CAT_COUNT_MIN, CAT_COUNT_MAX + 1);
+    int expectedCount = static_cast<int>(std::floor(countDraw));
+    if (expectedCount < CAT_COUNT_MIN) expectedCount = CAT_COUNT_MIN;
+    if (expectedCount > CAT_COUNT_MAX) expectedCount = CAT_COUNT_MAX;
+    REQUIRE(count_kind(sim, EntityKind::Cat) == expectedCount);
+
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Cat) continue;
+        const double margin = CAT_BODY_RADIUS + 8.0;
+        const double expectedX = prng_uniform(side, margin, 1920.0 - margin);
+        const double expectedSpeed = prng_uniform(side, CAT_WALK_SPEED_MIN, CAT_WALK_SPEED_MAX);
+        const double dirCoin = prng_uniform(side, 0.0, 1.0);
+        const double expectedDir = (dirCoin < 0.5) ? -1.0 : 1.0;
+        const uint32_t expectedSeed = prng_next_u32(side);
+        const double expectedTimer = prng_uniform(side, CAT_WALK_DURATION_MIN, CAT_WALK_DURATION_MAX);
+        const uint8_t expectedNameIndex = static_cast<uint8_t>(prng_index(side,
+            static_cast<uint32_t>(sizeof(CAT_NAME_POOL) / sizeof(CAT_NAME_POOL[0]))));
+        const uint8_t expectedCoatVariantIndex = static_cast<uint8_t>(prng_index(side,
+            static_cast<uint32_t>(CAT_COAT_VARIANT_COUNT)));
+
+        REQUIRE(e.x == Approx(expectedX));
+        REQUIRE(e.vx == Approx(expectedSpeed * expectedDir));
+        REQUIRE(e.seed == expectedSeed);
+        REQUIRE(e.stateTimer == Approx(expectedTimer));
+        REQUIRE(e.nameIndex == expectedNameIndex);
+        REQUIRE(e.coatVariantIndex == expectedCoatVariantIndex);
+        ++seen;
+    }
+    REQUIRE(seen == expectedCount);
+}
+
+TEST_CASE("sim_set_critter(None) clears ambient cats", "[cat][toggle]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+    REQUIRE(count_kind(sim, EntityKind::Cat) >= CAT_COUNT_MIN);
+
+    sim_set_critter(sim, CritterKind::None);
+    REQUIRE(sim.currentCritter == CritterKind::None);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Bunny) == 0);
+}
+
+TEST_CASE("Switching between critter species replaces the previous species", "[cat][toggle]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+    REQUIRE(count_kind(sim, EntityKind::Cat) >= CAT_COUNT_MIN);
+    REQUIRE(count_kind(sim, EntityKind::Sheep) == 0);
+
+    sim_set_critter(sim, CritterKind::Sheep);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Sheep) >= SHEEP_COUNT_MIN);
+
+    sim_set_critter(sim, CritterKind::Cat);
+    REQUIRE(count_kind(sim, EntityKind::Sheep) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Cat) >= CAT_COUNT_MIN);
+}
+
+TEST_CASE("sim_set_scene gates active Cat to Grass", "[cat][scene]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+    const int catsGrass = count_kind(sim, EntityKind::Cat);
+    REQUIRE(catsGrass >= CAT_COUNT_MIN);
+
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE(sim.currentCritter == CritterKind::Cat);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 0);
+
+    sim_set_scene(sim, Scene::Winter);
+    REQUIRE(sim.currentCritter == CritterKind::Cat);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 0);
+
+    sim_set_scene(sim, Scene::Grass);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == catsGrass);
+}
+
+TEST_CASE("Click within CAT_POUNCE_RADIUS pounces toward the click", "[cat][click]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+    keep_first_cat_only(sim);
+
+    Entity& cat = sim.entities.front();
+    cat.x = 500.0;
+    cat.vx = -CAT_WALK_SPEED_MIN;
+    cat.age = 5.0;
+
+    sim_apply_click(sim, click_event(cat.x + 16.0, sim.windowHeight - 20.0));
+
+    const Entity& after = sim.entities.front();
+    REQUIRE(after.state == CAT_STATE_POUNCING);
+    REQUIRE(after.stateTimer == Approx(CAT_POUNCE_DURATION));
+    REQUIRE(after.age == Approx(0.0));
+    REQUIRE(after.vx == Approx(CAT_POUNCE_SPEED));
+}
+
+TEST_CASE("Click outside CAT_POUNCE_RADIUS leaves cat alone", "[cat][click]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+    keep_first_cat_only(sim);
+
+    Entity& cat = sim.entities.front();
+    cat.x = 500.0;
+    cat.vx = -CAT_WALK_SPEED_MIN;
+    const uint8_t stateBefore = cat.state;
+    const double vxBefore = cat.vx;
+
+    sim_apply_click(sim, click_event(cat.x + CAT_POUNCE_RADIUS + 5.0, sim.windowHeight - 20.0));
+
+    const Entity& after = sim.entities.front();
+    REQUIRE(after.state == stateBefore);
+    REQUIRE(after.vx == Approx(vxBefore));
+}
+
+TEST_CASE("Cats do not greet other cats", "[cat][greeting]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+    keep_first_cat_only(sim);
+
+    Entity first = sim.entities.front();
+    first.x = 400.0;
+    first.vx = CAT_WALK_SPEED_MIN;
+    first.state = CAT_STATE_WALKING;
+    first.stateTimer = 10.0;
+    first.age = SHEEP_GREET_MIN_AGE + 1.0;
+    Entity second = first;
+    second.x = first.x + 20.0;
+    second.vx = -CAT_WALK_SPEED_MIN;
+    sim.entities.clear();
+    sim.entities.push_back(first);
+    sim.entities.push_back(second);
+
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 2);
+    for (const Entity& e : sim.entities) {
+        if (e.kind == EntityKind::Cat) REQUIRE(e.state != SHEEP_STATE_GREETING);
+    }
+}
+
+TEST_CASE("Cats do not greet sheep", "[cat][greeting]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Cat);
+    keep_first_cat_only(sim);
+
+    Entity cat = sim.entities.front();
+    cat.x = 400.0;
+    cat.vx = CAT_WALK_SPEED_MIN;
+    cat.state = CAT_STATE_WALKING;
+    cat.stateTimer = 10.0;
+    cat.age = SHEEP_GREET_MIN_AGE + 1.0;
+
+    Entity sheep{};
+    sheep.kind = EntityKind::Sheep;
+    sheep.size = SHEEP_BODY_RADIUS;
+    sheep.x = cat.x + 20.0;
+    sheep.y = sim.windowHeight - SHEEP_BODY_HEIGHT - SHEEP_LEG_LENGTH;
+    sheep.vx = -SHEEP_WALK_SPEED_MIN;
+    sheep.age = SHEEP_GREET_MIN_AGE + 1.0;
+    sheep.lifetime = -1.0;
+    sheep.state = SHEEP_STATE_WALKING;
+    sheep.stateTimer = 10.0;
+
+    sim.entities.clear();
+    sim.entities.push_back(cat);
+    sim.entities.push_back(sheep);
+
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 1);
+    REQUIRE(count_kind(sim, EntityKind::Sheep) == 1);
+    for (const Entity& e : sim.entities) REQUIRE(e.state != SHEEP_STATE_GREETING);
+}
+

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/click_through_smoke_test.cpp

@@ -0,0 +1,176 @@
+#include "../third_party/catch2/catch.hpp"
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+#include <atomic>
+#include <chrono>
+#include <string>
+#include <thread>
+
+namespace {
+
+std::atomic<bool> g_probeReceivedLeftDown{false};
+
+enum class ClickThroughResult {
+    Passed,
+    Skipped,
+    Failed,
+};
+
+std::wstring unique_class_name(const wchar_t* suffix) {
+    return std::wstring(L"DesktopGrass.Native.ClickThrough.")
+        + std::to_wstring(GetCurrentProcessId()) + L"."
+        + std::to_wstring(GetTickCount64()) + L"."
+        + suffix;
+}
+
+LRESULT CALLBACK ProbeWndProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp) {
+    if (msg == WM_LBUTTONDOWN) {
+        g_probeReceivedLeftDown.store(true, std::memory_order_release);
+    }
+    return DefWindowProcW(hwnd, msg, wp, lp);
+}
+
+LRESULT CALLBACK OverlayWndProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp) {
+    return DefWindowProcW(hwnd, msg, wp, lp);
+}
+
+void pump_messages_for(std::chrono::milliseconds duration) {
+    const auto deadline = std::chrono::steady_clock::now() + duration;
+    MSG msg{};
+    do {
+        while (PeekMessageW(&msg, nullptr, 0, 0, PM_REMOVE)) {
+            TranslateMessage(&msg);
+            DispatchMessageW(&msg);
+        }
+        if (g_probeReceivedLeftDown.load(std::memory_order_acquire)) {
+            return;
+        }
+        std::this_thread::sleep_for(std::chrono::milliseconds(5));
+    } while (std::chrono::steady_clock::now() < deadline);
+}
+
+bool has_interactive_desktop() {
+    if (GetConsoleWindow() == nullptr) {
+        return false;
+    }
+
+    HDESK inputDesktop = OpenInputDesktop(0, FALSE, DESKTOP_SWITCHDESKTOP);
+    if (inputDesktop == nullptr) {
+        return false;
+    }
+    CloseDesktop(inputDesktop);
+    return true;
+}
+
+ClickThroughResult spawn_probe_window_and_click_through_overlay() {
+    if (!has_interactive_desktop()) {
+        return ClickThroughResult::Skipped;
+    }
+
+    g_probeReceivedLeftDown.store(false, std::memory_order_release);
+
+    const HINSTANCE instance = GetModuleHandleW(nullptr);
+    const std::wstring probeClass = unique_class_name(L"Probe");
+    const std::wstring overlayClass = unique_class_name(L"Overlay");
+
+    WNDCLASSEXW probeWc{};
+    probeWc.cbSize = sizeof(probeWc);
+    probeWc.lpfnWndProc = ProbeWndProc;
+    probeWc.hInstance = instance;
+    probeWc.lpszClassName = probeClass.c_str();
+
+    WNDCLASSEXW overlayWc{};
+    overlayWc.cbSize = sizeof(overlayWc);
+    overlayWc.lpfnWndProc = OverlayWndProc;
+    overlayWc.hInstance = instance;
+    overlayWc.lpszClassName = overlayClass.c_str();
+
+    if (!RegisterClassExW(&probeWc)) {
+        return ClickThroughResult::Failed;
+    }
+    if (!RegisterClassExW(&overlayWc)) {
+        UnregisterClassW(probeClass.c_str(), instance);
+        return ClickThroughResult::Failed;
+    }
+
+    const int x = GetSystemMetrics(SM_XVIRTUALSCREEN) + 96;
+    const int y = GetSystemMetrics(SM_YVIRTUALSCREEN) + 96;
+    constexpr int kWidth = 96;
+    constexpr int kHeight = 64;
+    const int clickX = x + 24;
+    const int clickY = y + 24;
+
+    HWND probe = CreateWindowExW(
+        WS_EX_TOOLWINDOW | WS_EX_TOPMOST,
+        probeClass.c_str(), L"DesktopGrass click-through probe",
+        WS_POPUP | WS_VISIBLE,
+        x, y, kWidth, kHeight,
+        nullptr, nullptr, instance, nullptr);
+
+    HWND overlay = nullptr;
+    bool ok = probe != nullptr;
+    if (ok) {
+        SetWindowPos(probe, HWND_TOPMOST, x, y, kWidth, kHeight, SWP_SHOWWINDOW);
+
+        overlay = CreateWindowExW(
+            WS_EX_LAYERED | WS_EX_TRANSPARENT | WS_EX_TOPMOST |
+                WS_EX_TOOLWINDOW | WS_EX_NOACTIVATE,
+            overlayClass.c_str(), L"DesktopGrass click-through overlay",
+            WS_POPUP,
+            x, y, kWidth, kHeight,
+            nullptr, nullptr, instance, nullptr);
+        ok = overlay != nullptr;
+    }
+
+    if (ok) {
+        SetLayeredWindowAttributes(overlay, 0, 1, LWA_ALPHA);
+        ShowWindow(overlay, SW_SHOWNOACTIVATE);
+        SetWindowPos(overlay, HWND_TOPMOST, x, y, kWidth, kHeight,
+                     SWP_SHOWWINDOW | SWP_NOACTIVATE);
+        pump_messages_for(std::chrono::milliseconds(50));
+
+        if (!SetCursorPos(clickX, clickY)) {
+            ok = false;
+        }
+    }
+
+    ClickThroughResult result = ClickThroughResult::Failed;
+    if (ok) {
+        INPUT inputs[2]{};
+        inputs[0].type = INPUT_MOUSE;
+        inputs[0].mi.dwFlags = MOUSEEVENTF_LEFTDOWN;
+        inputs[1].type = INPUT_MOUSE;
+        inputs[1].mi.dwFlags = MOUSEEVENTF_LEFTUP;
+
+        const UINT sent = SendInput(2, inputs, sizeof(INPUT));
+        if (sent != 2) {
+            result = ClickThroughResult::Skipped;
+        } else {
+            pump_messages_for(std::chrono::milliseconds(200));
+            result = g_probeReceivedLeftDown.load(std::memory_order_acquire)
+                ? ClickThroughResult::Passed
+                : ClickThroughResult::Failed;
+        }
+    }
+
+    if (overlay) DestroyWindow(overlay);
+    if (probe) DestroyWindow(probe);
+    UnregisterClassW(overlayClass.c_str(), instance);
+    UnregisterClassW(probeClass.c_str(), instance);
+    return result;
+}
+
+} // namespace
+
+TEST_CASE("Overlay click-through allows input to reach windows beneath", "[smoke][input]") {
+    const ClickThroughResult result = spawn_probe_window_and_click_through_overlay();
+    if (result == ClickThroughResult::Skipped) {
+        WARN("Skipping click-through smoke test: requires an interactive desktop and SendInput.");
+        SUCCEED("Requires interactive session");
+        return;
+    }
+
+    REQUIRE(result == ClickThroughResult::Passed);
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/config_tests.cpp

@@ -0,0 +1,154 @@
+#include "../third_party/catch2/catch.hpp"
+#include "Config.h"
+
+#include <filesystem>
+#include <fstream>
+#include <sstream>
+#include <string>
+
+using namespace desktopgrass;
+
+namespace {
+
+std::filesystem::path test_config_path(const char* name) {
+    std::filesystem::path dir = std::filesystem::current_path()
+        / ".copilot-scratch"
+        / "native-config-tests"
+        / name;
+    std::error_code ec;
+    std::filesystem::remove_all(dir, ec);
+    std::filesystem::create_directories(dir);
+    return dir / "config.json";
+}
+
+void write_text(const std::filesystem::path& path, const std::string& text) {
+    std::filesystem::create_directories(path.parent_path());
+    std::ofstream file(path, std::ios::binary | std::ios::trunc);
+    file << text;
+}
+
+std::string read_text(const std::filesystem::path& path) {
+    std::ifstream file(path, std::ios::binary);
+    std::ostringstream buffer;
+    buffer << file.rdbuf();
+    return buffer.str();
+}
+
+} // namespace
+
+TEST_CASE("Config: missing file yields defaults and writes a template", "[config]") {
+    const std::filesystem::path path = test_config_path("missing");
+    REQUIRE_FALSE(std::filesystem::exists(path));
+
+    const config::Config cfg = config::LoadConfig(path.wstring());
+
+    CHECK(cfg.targetFps == config::kTargetFpsDefault);
+    CHECK(cfg.bladeDensity == Approx(config::kBladeDensityDefault));
+
+    // A default file should have been created and be re-readable (it is JSONC).
+    REQUIRE(std::filesystem::exists(path));
+    const config::Config reread = config::LoadConfig(path.wstring());
+    CHECK(reread.targetFps == config::kTargetFpsDefault);
+    CHECK(reread.bladeDensity == Approx(config::kBladeDensityDefault));
+}
+
+TEST_CASE("Config: valid values are parsed", "[config]") {
+    const std::filesystem::path path = test_config_path("valid");
+    write_text(path, "{ \"version\": 1, \"targetFps\": 60, \"bladeDensity\": 1.5 }");
+
+    const config::Config cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.targetFps == 60);
+    CHECK(cfg.bladeDensity == Approx(1.5));
+}
+
+TEST_CASE("Config: out-of-range values are clamped", "[config]") {
+    const std::filesystem::path path = test_config_path("clamp");
+    write_text(path, "{ \"targetFps\": 1000, \"bladeDensity\": 99.0 }");
+
+    config::Config cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.targetFps == config::kTargetFpsMax);
+    CHECK(cfg.bladeDensity == Approx(config::kBladeDensityMax));
+
+    write_text(path, "{ \"targetFps\": 0, \"bladeDensity\": 0.0 }");
+    cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.targetFps == config::kTargetFpsMin);
+    CHECK(cfg.bladeDensity == Approx(config::kBladeDensityMin));
+}
+
+TEST_CASE("Config: JSONC comments and trailing commas are tolerated", "[config]") {
+    const std::filesystem::path path = test_config_path("jsonc");
+    write_text(path,
+        "{\n"
+        "  // a comment\n"
+        "  \"targetFps\": 24, /* inline */\n"
+        "  \"bladeDensity\": 2.0,\n"  // trailing comma below
+        "}\n");
+
+    const config::Config cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.targetFps == 24);
+    CHECK(cfg.bladeDensity == Approx(2.0));
+}
+
+TEST_CASE("Config: malformed file falls back to defaults and is preserved", "[config]") {
+    const std::filesystem::path path = test_config_path("malformed");
+    write_text(path, "{ not valid json ");
+
+    const config::Config cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.targetFps == config::kTargetFpsDefault);
+    CHECK(cfg.bladeDensity == Approx(config::kBladeDensityDefault));
+
+    // The user's (broken) file must be left untouched for them to fix.
+    CHECK(read_text(path) == "{ not valid json ");
+}
+
+TEST_CASE("Config: missing keys fall back to per-key defaults", "[config]") {
+    const std::filesystem::path path = test_config_path("partial");
+    write_text(path, "{ \"targetFps\": 45 }");
+
+    const config::Config cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.targetFps == 45);
+    CHECK(cfg.bladeDensity == Approx(config::kBladeDensityDefault));
+    CHECK(cfg.swaySpeed == Approx(config::kSwaySpeedDefault));
+    CHECK(cfg.swayAmplitude == Approx(config::kSwayAmplitudeDefault));
+}
+
+TEST_CASE("Config: keys are matched case-insensitively", "[config]") {
+    const std::filesystem::path path = test_config_path("case-insensitive");
+    write_text(path,
+        "{ \"TargetFps\": 60, \"BLADEDENSITY\": 1.5, "
+        "\"SwaySpeed\": 0.5, \"swayamplitude\": 2.0 }");
+
+    const config::Config cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.targetFps == 60);
+    CHECK(cfg.bladeDensity == Approx(1.5));
+    CHECK(cfg.swaySpeed == Approx(0.5));
+    CHECK(cfg.swayAmplitude == Approx(2.0));
+}
+
+TEST_CASE("Config: sway knobs parse, clamp, and reject non-finite", "[config]") {
+    const std::filesystem::path path = test_config_path("sway");
+
+    // Defaults when absent.
+    write_text(path, "{ }");
+    config::Config cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.swaySpeed == Approx(config::kSwaySpeedDefault));
+    CHECK(cfg.swayAmplitude == Approx(config::kSwayAmplitudeDefault));
+
+    // Valid values parsed.
+    write_text(path, "{ \"swaySpeed\": 0.5, \"swayAmplitude\": 2.0 }");
+    cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.swaySpeed == Approx(0.5));
+    CHECK(cfg.swayAmplitude == Approx(2.0));
+
+    // Out-of-range clamped to bounds.
+    write_text(path, "{ \"swaySpeed\": 99.0, \"swayAmplitude\": -5.0 }");
+    cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.swaySpeed == Approx(config::kSwaySpeedMax));
+    CHECK(cfg.swayAmplitude == Approx(config::kSwayAmplitudeMin));
+
+    // Non-finite (inf from overflow) falls back to default, never poisons the sim.
+    write_text(path, "{ \"swaySpeed\": 1e999, \"swayAmplitude\": 1e999 }");
+    cfg = config::LoadConfig(path.wstring());
+    CHECK(cfg.swaySpeed == Approx(config::kSwaySpeedDefault));
+    CHECK(cfg.swayAmplitude == Approx(config::kSwayAmplitudeDefault));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/critter_tests.cpp

@@ -0,0 +1,379 @@
+// critter_tests.cpp
+//
+// Critter subsystem tests (architecture.md §13.3 / §16). Orthogonal to Scene.
+//
+// Coverage:
+//   * CritterKind discriminants are spec-locked ({None=0, Sheep=1}).
+//   * EntityKind::Sheep == 3 (added after the original {None, Tumbleweed,
+//     Snowflake} enum).
+//   * SHEEP_* and CRITTER_* constants are pinned to spec values.
+//   * sim_init defaults sim.currentCritter to None (no sheep until the user
+//     opts in via tray).
+//   * sim_set_critter(Sheep) on CANONICAL_TEST_SEED + 1920 produces
+//     deterministic count K ∈ [SHEEP_COUNT_MIN, SHEEP_COUNT_MAX], with
+//     every sheep entity well-formed: kind=Sheep, state=Walking, stateTimer
+//     in [WALK_DURATION_MIN, MAX], speed in [WALK_SPEED_MIN, MAX], x within
+//     monitor margins.
+//   * sim_set_critter(None) erases all sheep but preserves scene entities
+//     (snowflakes/tumbleweeds aren't touched).
+//   * sim_set_scene preserves the active critter — flipping Grass→Desert
+//     re-spawns sheep on the new scene.
+//   * Sheep PRNG draw order is bit-identical to a side-stream Prng for the
+//     locked sequence (count, then per-sheep: x, speed, dir-coin, seed,
+//     stateTimer, nameIndex).
+//   * Click within SHEEP_STARTLE_RADIUS pushes a sheep into Hopping, flips
+//     vx away from the cursor, and resets age.
+//   * Click outside SHEEP_STARTLE_RADIUS leaves sheep state untouched.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cwchar>
+
+using namespace desktopgrass;
+
+namespace {
+
+int count_kind(const Sim& sim, EntityKind kind) {
+    int n = 0;
+    for (const Entity& e : sim.entities) if (e.kind == kind) ++n;
+    return n;
+}
+
+int count_sheep(const Sim& sim) {
+    return count_kind(sim, EntityKind::Sheep);
+}
+
+const Entity* first_sheep(const Sim& sim) {
+    for (const Entity& e : sim.entities) if (e.kind == EntityKind::Sheep) return &e;
+    return nullptr;
+}
+
+} // namespace
+
+TEST_CASE("CritterKind has spec-locked discriminants", "[critter][enum]") {
+    REQUIRE(static_cast<int>(CritterKind::None)  == 0);
+    REQUIRE(static_cast<int>(CritterKind::Sheep) == 1);
+    REQUIRE(static_cast<int>(CritterKind::Cat)   == 2);
+    REQUIRE(static_cast<int>(CritterKind::Bunny) == 3);
+    REQUIRE(static_cast<int>(EntityKind::Sheep)  == 3);
+    REQUIRE(static_cast<int>(EntityKind::Bunny)  == 6);
+    REQUIRE(static_cast<int>(EntityKind::Butterfly) == 7);
+    REQUIRE(static_cast<int>(EntityKind::Firefly) == 8);
+    REQUIRE(CRITTER_DEFAULT == CritterKind::None);
+}
+
+TEST_CASE("Sheep constants are pinned to spec values", "[critter][constants]") {
+    REQUIRE(SHEEP_COUNT_MIN      == 2);
+    REQUIRE(SHEEP_COUNT_MAX      == 3);
+    REQUIRE(sizeof(PET_COUNT_OPTIONS) / sizeof(PET_COUNT_OPTIONS[0]) == 6);
+    for (int i = 0; i < 6; ++i) REQUIRE(PET_COUNT_OPTIONS[i] == i + 1);
+    REQUIRE(PET_COUNT_DEFAULT_SHEEP == SHEEP_COUNT_MIN);
+    REQUIRE(PET_COUNT_DEFAULT_CAT == CAT_COUNT_MIN);
+    REQUIRE(PET_COUNT_MAX_PER_MONITOR == 6);
+    REQUIRE(sizeof(SHEEP_NAME_POOL) / sizeof(SHEEP_NAME_POOL[0]) == 8);
+    REQUIRE(sizeof(CAT_NAME_POOL) / sizeof(CAT_NAME_POOL[0]) == 8);
+    REQUIRE(std::wcscmp(SHEEP_NAME_POOL[0], L"Bessie") == 0);
+    REQUIRE(std::wcscmp(SHEEP_NAME_POOL[7], L"Hazel") == 0);
+    REQUIRE(std::wcscmp(CAT_NAME_POOL[0], L"Mittens") == 0);
+    REQUIRE(std::wcscmp(CAT_NAME_POOL[7], L"Juno") == 0);
+    REQUIRE(PET_NAME_HOVER_RADIUS == Approx(50.0));
+    REQUIRE(PET_NAME_FADE_DURATION == Approx(1.5));
+    REQUIRE(PET_NAME_FONT_SIZE == Approx(11.0));
+    REQUIRE(PET_NAME_OFFSET_Y == Approx(-8.0));
+    REQUIRE(PET_NAME_COLOR == 0xFFFFFFFFu);
+    REQUIRE(PET_NAME_SHADOW_COLOR == 0xC0000000u);
+    REQUIRE(SHEEP_WALK_SPEED_MIN == Approx(14.0));
+    REQUIRE(SHEEP_WALK_SPEED_MAX == Approx(26.0));
+    REQUIRE(SHEEP_BODY_RADIUS    == Approx(12.0));
+    REQUIRE(SHEEP_HEAD_RADIUS    == Approx(5.0));
+    REQUIRE(SHEEP_LEG_LENGTH     == Approx(5.5));
+
+    REQUIRE(SHEEP_STATE_WALKING  == 0);
+    REQUIRE(SHEEP_STATE_GRAZING  == 1);
+    REQUIRE(SHEEP_STATE_IDLE     == 2);
+    REQUIRE(SHEEP_STATE_SLEEPING == 3);
+    REQUIRE(SHEEP_STATE_HOPPING  == 4);
+
+    REQUIRE(SHEEP_HOP_DURATION   == Approx(0.55));
+    REQUIRE(SHEEP_HOP_HEIGHT     == Approx(11.0));
+    REQUIRE(SHEEP_STARTLE_RADIUS == Approx(64.0));
+    REQUIRE(SHEEP_STARTLE_BOOST  == Approx(1.6));
+
+    REQUIRE(SHEEP_GRAZE_PROBABILITY     == Approx(0.60));
+    REQUIRE(SHEEP_IDLE_PROBABILITY      == Approx(0.25));
+    REQUIRE(SHEEP_SLEEP_FROM_IDLE_PROB  == Approx(0.30));
+}
+
+TEST_CASE("sim_init defaults critter to None", "[critter][init]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    REQUIRE(sim.currentCritter == CritterKind::None);
+    REQUIRE(count_sheep(sim) == 0);
+}
+
+TEST_CASE("sim_set_critter(Sheep) produces deterministic flock", "[critter][gen]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+
+    REQUIRE(sim.currentCritter == CritterKind::Sheep);
+    const int k = count_sheep(sim);
+    REQUIRE(k >= SHEEP_COUNT_MIN);
+    REQUIRE(k <= SHEEP_COUNT_MAX);
+
+    const double groundY = sim.windowHeight;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Sheep) continue;
+        REQUIRE(e.state == SHEEP_STATE_WALKING);
+        REQUIRE(e.stateTimer >= SHEEP_WALK_DURATION_MIN);
+        REQUIRE(e.stateTimer <  SHEEP_WALK_DURATION_MAX);
+        REQUIRE(std::fabs(e.vx) >= SHEEP_WALK_SPEED_MIN);
+        REQUIRE(std::fabs(e.vx) <  SHEEP_WALK_SPEED_MAX);
+        const double margin = e.size + 8.0;
+        REQUIRE(e.x >= margin);
+        REQUIRE(e.x <= sim.monitorWidth - margin);
+        REQUIRE(e.y == Approx(groundY - SHEEP_BODY_HEIGHT - SHEEP_LEG_LENGTH));
+        REQUIRE(e.lifetime < 0.0); // infinite — sheep don't expire
+        REQUIRE(e.nameIndex < sizeof(SHEEP_NAME_POOL) / sizeof(SHEEP_NAME_POOL[0]));
+    }
+}
+
+TEST_CASE("Sheep PRNG draw order matches a side stream", "[critter][prng]") {
+    // Independent side stream that walks the documented sequence:
+    //   count
+    //   per-sheep: x, speed, dir-coin, seed, stateTimer, nameIndex
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+
+    const double countDraw = prng_uniform(side, SHEEP_COUNT_MIN, SHEEP_COUNT_MAX + 1);
+    int expectedCount = static_cast<int>(std::floor(countDraw));
+    if (expectedCount < SHEEP_COUNT_MIN) expectedCount = SHEEP_COUNT_MIN;
+    if (expectedCount > SHEEP_COUNT_MAX) expectedCount = SHEEP_COUNT_MAX;
+    REQUIRE(count_sheep(sim) == expectedCount);
+
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Sheep) continue;
+        const double margin = SHEEP_BODY_RADIUS + 8.0;
+        const double expectedX = prng_uniform(side, margin, 1920.0 - margin);
+        const double expectedSpeed = prng_uniform(side, SHEEP_WALK_SPEED_MIN, SHEEP_WALK_SPEED_MAX);
+        const double dirCoin = prng_uniform(side, 0.0, 1.0);
+        const double expectedDir = (dirCoin < 0.5) ? -1.0 : 1.0;
+        const uint32_t expectedSeed = prng_next_u32(side);
+        const double expectedTimer = prng_uniform(side, SHEEP_WALK_DURATION_MIN, SHEEP_WALK_DURATION_MAX);
+        const uint8_t expectedNameIndex = static_cast<uint8_t>(prng_index(side,
+            static_cast<uint32_t>(sizeof(SHEEP_NAME_POOL) / sizeof(SHEEP_NAME_POOL[0]))));
+
+        REQUIRE(e.x == Approx(expectedX));
+        REQUIRE(e.vx == Approx(expectedSpeed * expectedDir));
+        REQUIRE(e.seed == expectedSeed);
+        REQUIRE(e.stateTimer == Approx(expectedTimer));
+        REQUIRE(e.nameIndex == expectedNameIndex);
+        ++seen;
+    }
+    REQUIRE(seen == expectedCount);
+}
+
+TEST_CASE("canonical critter name indices are stable and species-local", "[critter][names]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+    const uint8_t expectedSheepNames[] = { 4, 7 };
+    int sheepSeen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Sheep) continue;
+        REQUIRE(sheepSeen < static_cast<int>(sizeof(expectedSheepNames) / sizeof(expectedSheepNames[0])));
+        REQUIRE(e.nameIndex == expectedSheepNames[sheepSeen]);
+        REQUIRE(std::wcscmp(SHEEP_NAME_POOL[e.nameIndex], sheepSeen == 0 ? L"Pippin" : L"Hazel") == 0);
+        ++sheepSeen;
+    }
+    REQUIRE(sheepSeen == 2);
+
+    sim_set_critter(sim, CritterKind::Cat);
+    const uint8_t expectedCatNames[] = { 4 };
+    int catSeen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Cat) continue;
+        REQUIRE(catSeen < static_cast<int>(sizeof(expectedCatNames) / sizeof(expectedCatNames[0])));
+        REQUIRE(e.nameIndex == expectedCatNames[catSeen]);
+        REQUIRE(e.nameIndex < sizeof(CAT_NAME_POOL) / sizeof(CAT_NAME_POOL[0]));
+        REQUIRE(std::wcscmp(CAT_NAME_POOL[e.nameIndex], L"Smokey") == 0);
+        ++catSeen;
+    }
+    REQUIRE(catSeen == 1);
+}
+
+TEST_CASE("sim_set_critter_count(0) preserves random sheep count draw", "[critter][count]") {
+    bool sawMin = false;
+    bool sawMax = false;
+    for (uint64_t i = 0; i < 64; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i * 0x9E3779B97F4A7C15ull;
+        Sim sim = sim_init(seed, 1920.0, DEFAULT_DENSITY);
+        sim_set_critter_count(sim, 3);
+        sim_set_critter_count(sim, 0);
+        sim_set_critter(sim, CritterKind::Sheep);
+
+        Prng side;
+        prng_init(side, seed ^ CRITTER_PRNG_SALT);
+        const double countDraw = prng_uniform(side, SHEEP_COUNT_MIN, SHEEP_COUNT_MAX + 1);
+        int expectedCount = static_cast<int>(std::floor(countDraw));
+        if (expectedCount < SHEEP_COUNT_MIN) expectedCount = SHEEP_COUNT_MIN;
+        if (expectedCount > SHEEP_COUNT_MAX) expectedCount = SHEEP_COUNT_MAX;
+
+        REQUIRE(count_sheep(sim) == expectedCount);
+        sawMin = sawMin || expectedCount == SHEEP_COUNT_MIN;
+        sawMax = sawMax || expectedCount == SHEEP_COUNT_MAX;
+    }
+    REQUIRE(sawMin);
+    REQUIRE(sawMax);
+}
+
+TEST_CASE("fixed sheep count override skips the count PRNG draw", "[critter][count][prng]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+    sim_set_critter_count(sim, 3);
+
+    REQUIRE(sim.critterCountOverride == 3);
+    REQUIRE(count_sheep(sim) == 3);
+
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Sheep) continue;
+        const double margin = SHEEP_BODY_RADIUS + 8.0;
+        const double expectedX = prng_uniform(side, margin, 1920.0 - margin);
+        const double expectedSpeed = prng_uniform(side, SHEEP_WALK_SPEED_MIN, SHEEP_WALK_SPEED_MAX);
+        const double dirCoin = prng_uniform(side, 0.0, 1.0);
+        const double expectedDir = (dirCoin < 0.5) ? -1.0 : 1.0;
+        const uint32_t expectedSeed = prng_next_u32(side);
+        const double expectedTimer = prng_uniform(side, SHEEP_WALK_DURATION_MIN, SHEEP_WALK_DURATION_MAX);
+        const uint8_t expectedNameIndex = static_cast<uint8_t>(prng_index(side,
+            static_cast<uint32_t>(sizeof(SHEEP_NAME_POOL) / sizeof(SHEEP_NAME_POOL[0]))));
+
+        REQUIRE(e.x == Approx(expectedX));
+        REQUIRE(e.vx == Approx(expectedSpeed * expectedDir));
+        REQUIRE(e.seed == expectedSeed);
+        REQUIRE(e.stateTimer == Approx(expectedTimer));
+        REQUIRE(e.nameIndex == expectedNameIndex);
+        ++seen;
+    }
+    REQUIRE(seen == 3);
+}
+
+TEST_CASE("fixed critter count override supports tray range and clamps", "[critter][count]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+
+    sim_set_critter_count(sim, 6);
+    REQUIRE(count_sheep(sim) == 6);
+
+    sim_set_critter_count(sim, 8);
+    REQUIRE(count_sheep(sim) == PET_COUNT_MAX_PER_MONITOR);
+
+    sim_set_critter(sim, CritterKind::Cat);
+    sim_set_critter_count(sim, 2);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 2);
+    REQUIRE(count_sheep(sim) == 0);
+}
+
+TEST_CASE("sim_set_critter(None) clears all ground critters",
+          "[critter][toggle]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+    REQUIRE(count_sheep(sim) >= SHEEP_COUNT_MIN);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 0);
+
+    sim_set_critter(sim, CritterKind::None);
+    REQUIRE(count_sheep(sim) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Cat) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Bunny) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Hedgehog) == 0);
+}
+
+TEST_CASE("sim_set_scene gates active sheep to Grass", "[critter][scene]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+    const int sheepCountGrass = count_sheep(sim);
+    REQUIRE(sheepCountGrass >= SHEEP_COUNT_MIN);
+
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE(count_sheep(sim) == 0);
+    REQUIRE(sim.currentCritter == CritterKind::Sheep);
+
+    sim_set_scene(sim, Scene::Winter);
+    REQUIRE(count_sheep(sim) == 0);
+    REQUIRE(sim.currentCritter == CritterKind::Sheep);
+
+    sim_set_scene(sim, Scene::Grass);
+    REQUIRE(count_sheep(sim) == sheepCountGrass);
+}
+
+TEST_CASE("Click within SHEEP_STARTLE_RADIUS triggers hop away", "[critter][click]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+
+    Entity* target = nullptr;
+    for (Entity& e : sim.entities) {
+        if (e.kind == EntityKind::Sheep) { target = &e; break; }
+    }
+    REQUIRE(target != nullptr);
+
+    // Click 16 DIP to the left of the sheep — well within startle radius,
+    // inside the cut band (so the early y-gate doesn't reject).
+    const double clickX = target->x - 16.0;
+    const double clickY = sim.windowHeight - 20.0;
+    target->age = 5.0; // pre-set age to verify reset
+
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x = clickX;
+    ev.y = clickY;
+    ev.time = 0.0;
+    sim_apply_click(sim, ev);
+
+    Entity* after = nullptr;
+    for (Entity& e : sim.entities) {
+        if (e.kind == EntityKind::Sheep) { after = &e; break; }
+    }
+    REQUIRE(after != nullptr);
+    REQUIRE(after->state == SHEEP_STATE_HOPPING);
+    REQUIRE(after->stateTimer == Approx(SHEEP_HOP_DURATION));
+    REQUIRE(after->age == Approx(0.0));
+    REQUIRE(after->vx > 0.0); // sheep was right of click → vx flipped to +
+    REQUIRE(std::fabs(after->vx) <= SHEEP_WALK_SPEED_MAX * SHEEP_STARTLE_BOOST);
+}
+
+TEST_CASE("Click outside SHEEP_STARTLE_RADIUS leaves sheep alone", "[critter][click]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+
+    Entity* target = nullptr;
+    for (Entity& e : sim.entities) {
+        if (e.kind == EntityKind::Sheep) { target = &e; break; }
+    }
+    REQUIRE(target != nullptr);
+    const uint8_t stateBefore = target->state;
+    const double  vxBefore    = target->vx;
+
+    // Click far away (300 DIP) but still in the cut band.
+    const double clickX = target->x + SHEEP_STARTLE_RADIUS + 200.0;
+    const double clickY = sim.windowHeight - 20.0;
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x = clickX;
+    ev.y = clickY;
+    ev.time = 0.0;
+    sim_apply_click(sim, ev);
+
+    Entity* after = nullptr;
+    for (Entity& e : sim.entities) {
+        if (e.kind == EntityKind::Sheep) { after = &e; break; }
+    }
+    REQUIRE(after != nullptr);
+    REQUIRE(after->state == stateBefore);
+    REQUIRE(after->vx == Approx(vxBefore));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/cut_tests.cpp

@@ -0,0 +1,259 @@
+// cut_tests.cpp
+//
+// Cut state animation tests (architecture.md §9).
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+#include "snapshot_data.h"
+
+#include <cmath>
+#include <vector>
+
+using namespace desktopgrass;
+using namespace desktopgrass::test;
+
+namespace {
+
+Sim make_sim_with_blades(std::initializer_list<double> baseXs) {
+    Sim sim;
+    sim.windowHeight = STRIP_HEIGHT + HEADROOM;
+    for (double x : baseXs) {
+        Blade b{};
+        b.baseX            = x;
+        b.height           = 20.0;
+        b.thickness        = 1.5;
+        b.swayPhaseOffset  = 0.0;
+        b.stiffness        = 1.0;
+        b.cutHeight        = 1.0;
+        b.cutInitialHeight = 1.0;
+        b.cutAnimStart     = -1.0;
+        sim.blades.push_back(b);
+    }
+    return sim;
+}
+
+InputEvent click(double x, double y, double t) {
+    return InputEvent{ EventType::Click, x, y, t };
+}
+
+} // anonymous
+
+TEST_CASE("click inside cut band animates blades within radius to 0", "[cut]") {
+    Sim sim = make_sim_with_blades({100.0, 110.0, 200.0});
+    const double y_in_band = sim.windowHeight - 40.0; // inside strip
+
+    InputEvent ev = click(100.0, y_in_band, 0.0);
+    sim_tick(sim, 0.0, &ev, 1);
+
+    // Apply 5 ticks of 50 ms (total = 250 ms > CUT_DURATION_SEC).
+    for (int i = 0; i < 5; ++i) {
+        sim_tick(sim, 0.05, nullptr, 0);
+    }
+
+    REQUIRE(sim.blades[0].cutHeight == Approx(0.0));
+    REQUIRE(sim.blades[0].cutAnimStart == Approx(-1.0));
+    REQUIRE(sim.blades[1].cutHeight == Approx(0.0));
+    // Blade at 200 is outside CUT_RADIUS = 30.
+    REQUIRE(sim.blades[2].cutHeight == Approx(1.0));
+    REQUIRE(sim.blades[2].cutAnimStart == Approx(-1.0));
+}
+
+TEST_CASE("cut animation is linear over CUT_DURATION_SEC", "[cut]") {
+    Sim sim = make_sim_with_blades({100.0});
+    const double y = sim.windowHeight - 40.0;
+
+    InputEvent ev = click(100.0, y, 0.0);
+    sim_tick(sim, 0.0, &ev, 1);
+    // After tick(0.0) globalTime = 0 still; cutAnimStart = 0.
+
+    // 50 ms in → cutHeight ≈ 0.75.
+    sim_tick(sim, 0.05, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight == Approx(0.75).margin(1e-9));
+
+    // 100 ms in → 0.5.
+    sim_tick(sim, 0.05, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight == Approx(0.5).margin(1e-9));
+
+    // 150 ms in → 0.25.
+    sim_tick(sim, 0.05, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight == Approx(0.25).margin(1e-9));
+
+    // 200 ms in → 0.0 and idle.
+    sim_tick(sim, 0.05, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight == Approx(0.0).margin(1e-9));
+    REQUIRE(sim.blades[0].cutAnimStart < 0.0);
+}
+
+TEST_CASE("click outside cut band is ignored", "[cut]") {
+    Sim sim = make_sim_with_blades({100.0});
+    const double y_above = sim.windowHeight - STRIP_HEIGHT - 5.0;
+
+    InputEvent ev = click(100.0, y_above, 0.0);
+    sim_tick(sim, 0.0, &ev, 1);
+
+    REQUIRE(sim.blades[0].cutHeight == Approx(1.0));
+    REQUIRE(sim.blades[0].cutAnimStart < 0.0);
+}
+
+TEST_CASE("repeat click on in-flight blade is idempotent", "[cut]") {
+    Sim sim = make_sim_with_blades({100.0});
+    const double y = sim.windowHeight - 40.0;
+
+    InputEvent first = click(100.0, y, 0.0);
+    sim_tick(sim, 0.0, &first, 1);
+
+    // Mid-animation second click → should not reset cutAnimStart.
+    sim_tick(sim, 0.05, nullptr, 0);  // 0.05 elapsed; cutHeight = 0.75
+    const double startSnapshot = sim.blades[0].cutAnimStart;
+    const double heightSnapshot = sim.blades[0].cutHeight;
+
+    InputEvent second = click(100.0, y, 0.05);
+    sim_tick(sim, 0.0, &second, 1);
+
+    REQUIRE(sim.blades[0].cutAnimStart    == Approx(startSnapshot));
+    REQUIRE(sim.blades[0].cutInitialHeight == Approx(1.0));
+    REQUIRE(sim.blades[0].cutHeight == Approx(heightSnapshot));
+}
+
+TEST_CASE("click on already-cut blade is a no-op", "[cut]") {
+    Sim sim = make_sim_with_blades({100.0});
+    sim.blades[0].cutHeight        = 0.0;
+    sim.blades[0].cutInitialHeight = 0.0;
+    sim.blades[0].cutAnimStart     = -1.0;
+
+    const double y = sim.windowHeight - 40.0;
+    InputEvent ev = click(100.0, y, 0.0);
+    sim_tick(sim, 0.0, &ev, 1);
+
+    REQUIRE(sim.blades[0].cutHeight    == Approx(0.0));
+    REQUIRE(sim.blades[0].cutAnimStart  < 0.0);
+}
+
+TEST_CASE("blades outside cut radius are untouched", "[cut]") {
+    Sim sim = make_sim_with_blades({100.0, 131.0, 200.0});
+    const double y = sim.windowHeight - 40.0;
+
+    InputEvent ev = click(100.0, y, 0.0);
+    sim_tick(sim, 0.0, &ev, 1);
+
+    REQUIRE(sim.blades[0].cutAnimStart >= 0.0);
+    REQUIRE(sim.blades[1].cutAnimStart  < 0.0);
+    REQUIRE(sim.blades[2].cutAnimStart  < 0.0);
+}
+
+TEST_CASE("compute_blade_stroke degenerates to a stump under threshold", "[cut][geometry]") {
+    Blade b{};
+    b.baseX            = 100.0;
+    b.height           = 20.0;
+    b.thickness        = 1.5;
+    b.hue              = 2;
+    b.cutHeight        = 0.04;       // below CUT_STUMP_THRESHOLD = 0.05
+    b.effectiveLean    = 5.0;
+    b.cutInitialHeight = 1.0;
+    b.cutAnimStart     = 0.0;
+
+    Stroke s = compute_blade_stroke(b, 110.0, Scene::Grass);
+    REQUIRE(s.tip.x == Approx(100.0));
+    REQUIRE(s.tip.y == Approx(110.0 - STUMP_HEIGHT));
+    REQUIRE(s.argb  == PALETTE[2]);
+}
+
+TEST_CASE("compute_blade_stroke produces vertical line when lean is zero", "[cut][geometry]") {
+    Blade b{};
+    b.baseX         = 100.0;
+    b.height        = 20.0;
+    b.thickness     = 1.5;
+    b.hue           = 1;
+    b.cutHeight     = 1.0;
+    b.effectiveLean = 0.0;
+
+    Stroke s = compute_blade_stroke(b, 110.0, Scene::Grass);
+    REQUIRE(s.base.x    == Approx(100.0));
+    REQUIRE(s.base.y    == Approx(110.0));
+    REQUIRE(s.tip.x     == Approx(100.0));
+    REQUIRE(s.tip.y     == Approx(90.0));
+    REQUIRE(s.control.x == Approx(100.0));
+}
+
+// ---------------------------------------------------------------------------
+// Cut-floor (stubble) variation
+// ---------------------------------------------------------------------------
+
+TEST_CASE("generated blades get a per-blade cut floor within spec range", "[cut][floor]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    REQUIRE(blades.size() > 50);
+
+    for (const Blade& b : blades) {
+        REQUIRE(b.cutFloor >= CUT_FLOOR_MIN);
+        REQUIRE(b.cutFloor <  CUT_FLOOR_MAX);
+        // Stubble must render as a short blade, never a degenerate stump.
+        REQUIRE(b.cutFloor >= CUT_STUMP_THRESHOLD);
+    }
+
+    // The whole point is variation: not every blade settles at the same height.
+    bool varies = false;
+    for (std::size_t i = 1; i < blades.size(); ++i) {
+        if (blades[i].cutFloor != blades[0].cutFloor) { varies = true; break; }
+    }
+    REQUIRE(varies);
+}
+
+TEST_CASE("cut settles at the per-blade stubble floor, not flat zero", "[cut][floor]") {
+    Blade b{};
+    b.height           = 20.0;
+    b.thickness        = 1.5;
+    b.cutHeight        = 1.0;
+    b.cutInitialHeight = 1.0;
+    b.cutFloor         = 0.12;
+    b.cutAnimStart     = 0.0;
+
+    // Advance past the full cut duration.
+    advance_cut(b, CUT_DURATION_SEC + 0.01);
+
+    REQUIRE(b.cutHeight    == Approx(0.12));
+    REQUIRE(b.cutAnimStart == Approx(-1.0));
+}
+
+TEST_CASE("cut-down animation lerps toward the floor", "[cut][floor]") {
+    Blade b{};
+    b.height           = 20.0;
+    b.cutHeight        = 1.0;
+    b.cutInitialHeight = 1.0;
+    b.cutFloor         = 0.10;
+    b.cutAnimStart     = 0.0;
+
+    // Half-way through the cut: lerp(1.0 -> 0.10) at t=0.5 = 0.10 + 0.90*0.5.
+    advance_cut(b, CUT_DURATION_SEC * 0.5);
+    REQUIRE(b.cutHeight == Approx(0.10 + 0.90 * 0.5).margin(1e-9));
+}
+
+TEST_CASE("regrowth grows back from the floor to full height", "[cut][floor][regrowth]") {
+    Blade b{};
+    b.height         = 20.0;
+    b.cutFloor       = 0.10;
+    b.cutHeight      = 0.10;
+    b.cutAnimStart   = -1.0;
+    b.regrowDuration = 0.4;
+    b.regrowStart    = 0.0;
+
+    // Half-way through regrowth: lerp(0.10 -> 1.0) at t=0.5.
+    advance_cut(b, 0.2);
+    REQUIRE(b.cutHeight == Approx(0.10 + 0.90 * 0.5).margin(1e-9));
+
+    // Fully regrown.
+    advance_cut(b, 0.4);
+    REQUIRE(b.cutHeight == Approx(1.0).margin(1e-9));
+}
+
+TEST_CASE("zero-floor blades still collapse fully (back-compat)", "[cut][floor]") {
+    Blade b{};
+    b.height           = 20.0;
+    b.cutHeight        = 1.0;
+    b.cutInitialHeight = 1.0;
+    b.cutFloor         = 0.0;
+    b.cutAnimStart     = 0.0;
+
+    advance_cut(b, CUT_DURATION_SEC + 0.01);
+    REQUIRE(b.cutHeight == Approx(0.0));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/desert_tests.cpp

@@ -0,0 +1,220 @@
+// desert_tests.cpp - §14 Desert scene cacti + tumbleweeds.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+#include "snapshot_data.h"
+
+#include <cmath>
+#include <cstddef>
+#include <vector>
+
+using namespace desktopgrass;
+
+namespace {
+
+constexpr double kMonitor1920 = 1920.0;
+
+struct ExpectedCactus {
+    std::size_t slotIndex = 0;
+    uint8_t type = 0;
+    double height = 0.0;
+    double width = 0.0;
+    int8_t armSide = +1;
+};
+
+ExpectedCactus first_expected_cactus(std::size_t bladeCount) {
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED ^ CACTUS_PRNG_SALT);
+
+    for (std::size_t i = 0; i < bladeCount; ++i) {
+        const double r = prng_uniform(p, 0.0, 1.0);
+        if (r >= CACTUS_PROBABILITY) continue;
+
+        ExpectedCactus expected{};
+        expected.slotIndex = i;
+        expected.height = prng_uniform(p, CACTUS_HEIGHT_MIN, CACTUS_HEIGHT_MAX);
+        expected.width = prng_uniform(p, CACTUS_WIDTH_MIN, CACTUS_WIDTH_MAX);
+
+        const double armDraw = prng_uniform(p, 0.0, 1.0);
+        const double noArmThreshold = 1.0 - CACTUS_ARM_PROBABILITY;
+        const double twoArmThreshold = noArmThreshold + CACTUS_TWO_ARM_PROBABILITY * CACTUS_ARM_PROBABILITY;
+        if (armDraw < noArmThreshold) {
+            expected.type = 0;
+        } else if (armDraw < twoArmThreshold) {
+            expected.type = 2;
+        } else {
+            expected.type = 1;
+            expected.armSide = prng_uniform(p, 0.0, 1.0) < 0.5
+                ? static_cast<int8_t>(-1)
+                : static_cast<int8_t>(+1);
+        }
+        if (expected.height < CACTUS_ARM_MIN_HEIGHT) {
+            expected.type = 0;
+            expected.armSide = +1;
+        }
+        return expected;
+    }
+
+    FAIL("canonical seed produced no cactus slot");
+    return {};
+}
+
+int expected_tumbleweed_count(double monitorWidth) {
+    if (monitorWidth < 480.0) return 0;
+    int count = static_cast<int>(std::floor(monitorWidth / 1920.0 * static_cast<double>(TUMBLEWEED_COUNT_PER_1920DIP)));
+    return count < 1 ? 1 : count;
+}
+
+} // anonymous
+
+TEST_CASE("Desert constants are pinned", "[desert][constants]") {
+    REQUIRE(CACTUS_PROBABILITY == Approx(0.005));
+    REQUIRE(CACTUS_HEIGHT_MIN == Approx(30.0));
+    REQUIRE(CACTUS_HEIGHT_MAX == Approx(70.0));
+    REQUIRE(CACTUS_COLOR == 0xFF2D7A2Du);
+    REQUIRE(TUMBLEWEED_COUNT_PER_1920DIP == 4);
+    REQUIRE(TUMBLEWEED_SPEED_MAX == Approx(72.0));
+    REQUIRE(TUMBLEWEED_PRNG_SALT == 0x7B0117CA7B0117CAull);
+}
+
+TEST_CASE("sim_set_scene Desert clears entities and generates cacti", "[desert][scene]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    Entity fake{};
+    fake.kind = EntityKind::Snowflake;
+    sim.entities.push_back(fake);
+
+    sim_set_scene(sim, Scene::Desert);
+
+    REQUIRE(sim.currentScene == Scene::Desert);
+    REQUIRE(sim.entities.size() == static_cast<std::size_t>(expected_tumbleweed_count(kMonitor1920)));
+    for (const Entity& e : sim.entities) REQUIRE(e.kind == EntityKind::Tumbleweed);
+
+    std::size_t cactusCount = 0;
+    for (const Blade& b : sim.blades) if (b.isCactus) ++cactusCount;
+    REQUIRE(cactusCount >= 1);
+    REQUIRE(cactusCount <= 10);
+}
+
+TEST_CASE("First cactus matches the spec-derived PRNG snapshot", "[desert][cactus]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    const ExpectedCactus expected = first_expected_cactus(sim.blades.size());
+
+    sim_set_scene(sim, Scene::Desert);
+
+    REQUIRE(expected.slotIndex < sim.blades.size());
+    const Blade& b = sim.blades[expected.slotIndex];
+    REQUIRE(b.isCactus);
+    REQUIRE(b.cactusType == expected.type);
+    REQUIRE(b.cactusHeight == Approx(expected.height).margin(1e-12));
+    REQUIRE(b.cactusWidth == Approx(expected.width).margin(1e-12));
+    if (expected.type == 1) REQUIRE(b.cactusArmSide == expected.armSide);
+}
+
+TEST_CASE("Grass scene restores original flower and mushroom slot variants", "[desert][restore]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    const ExpectedCactus expected = first_expected_cactus(sim.blades.size());
+    REQUIRE(expected.slotIndex < sim.blades.size());
+
+    Blade& target = sim.blades[expected.slotIndex];
+    target.isFlower = true;
+    target.isMushroom = true;
+    target.originalIsFlower = true;
+    target.originalIsMushroom = true;
+
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE(sim.blades[expected.slotIndex].isCactus);
+    REQUIRE_FALSE(sim.blades[expected.slotIndex].isFlower);
+    REQUIRE_FALSE(sim.blades[expected.slotIndex].isMushroom);
+
+    sim_set_scene(sim, Scene::Grass);
+    REQUIRE_FALSE(sim.blades[expected.slotIndex].isCactus);
+    REQUIRE(sim.blades[expected.slotIndex].isFlower);
+    REQUIRE(sim.blades[expected.slotIndex].isMushroom);
+}
+
+TEST_CASE("Desert generates the expected tumbleweed count", "[desert][tumbleweed]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Desert);
+
+    const int expected = expected_tumbleweed_count(kMonitor1920);
+    REQUIRE(expected >= 1);
+    REQUIRE(sim.entities.size() == static_cast<std::size_t>(expected));
+}
+
+TEST_CASE("First tumbleweed matches the spec-derived PRNG snapshot", "[desert][tumbleweed]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE_FALSE(sim.entities.empty());
+
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED ^ TUMBLEWEED_PRNG_SALT);
+    const double expectedSize = prng_uniform(p, TUMBLEWEED_SIZE_MIN, TUMBLEWEED_SIZE_MAX);
+    const double expectedX = prng_uniform(p, 0.0, kMonitor1920);
+    const double expectedY = sim.windowHeight - prng_uniform(p, TUMBLEWEED_Y_OFFSET_MIN, TUMBLEWEED_Y_OFFSET_MAX);
+    const double speed = prng_uniform(p, TUMBLEWEED_SPEED_MIN, TUMBLEWEED_SPEED_MAX);
+    const double direction = prng_uniform(p, 0.0, 1.0) < 0.5 ? -1.0 : 1.0;
+    const double expectedVx = direction * speed;
+    const double expectedRotation = prng_uniform(p, 0.0, 6.28318530717958647692);
+
+    const Entity& e = sim.entities[0];
+    REQUIRE(e.kind == EntityKind::Tumbleweed);
+    REQUIRE(e.size == Approx(expectedSize).margin(1e-12));
+    REQUIRE(e.x == Approx(expectedX).margin(1e-12));
+    REQUIRE(e.y == Approx(expectedY).margin(1e-12));
+    REQUIRE(e.vx == Approx(expectedVx).margin(1e-12));
+    REQUIRE(e.rotation == Approx(expectedRotation).margin(1e-12));
+    REQUIRE(e.rotationSpeed == Approx(expectedVx / expectedSize).margin(1e-12));
+}
+
+TEST_CASE("Tumbleweed respawns at the opposite edge when off-screen", "[desert][tumbleweed]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE_FALSE(sim.entities.empty());
+
+    sim.entities[0].x = sim.monitorWidth + 100.0;
+    sim_tick_entities(sim, 0.0);
+
+    const Entity& e = sim.entities[0];
+    REQUIRE(e.kind == EntityKind::Tumbleweed);
+    REQUIRE(e.x == Approx(-e.size).margin(1e-12));
+    REQUIRE(e.vx > 0.0);
+}
+
+TEST_CASE("Tumbleweed hops above its baseline then settles", "[desert][tumbleweed]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE_FALSE(sim.entities.empty());
+
+    double yBase = sim.entities[0].altitudeAnchor;
+    REQUIRE(sim.entities[0].y == Approx(yBase).margin(1e-9)); // starts grounded
+
+    double minY = sim.entities[0].y;
+    for (int i = 0; i < 900; ++i) {
+        sim_tick_entities(sim, 1.0 / 60.0);
+        Entity& t = sim.entities[0];
+        // Pin x on-screen so it doesn't roll off and respawn mid-test.
+        if (t.x < 50.0) t.x = 50.0;
+        if (t.x > sim.monitorWidth - 50.0) t.x = sim.monitorWidth - 50.0;
+        yBase = t.altitudeAnchor;
+        minY = std::min(minY, t.y);
+        REQUIRE(t.y <= yBase + 1e-6); // never sinks below the baseline
+    }
+
+    REQUIRE(minY < yBase - 1.0); // it left the ground at least once
+}
+
+TEST_CASE("Desert scene leaves the canonical first blade geometry bit-identical", "[desert][snapshot]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, 1.0);
+    REQUIRE(sim.blades.size() == desktopgrass::test::CANONICAL_BLADE_COUNT);
+
+    sim_set_scene(sim, Scene::Desert);
+
+    const Blade& first = sim.blades[0];
+    const auto& expected = desktopgrass::test::CANONICAL_FIRST_10[0];
+    REQUIRE(first.baseX == Approx(expected.baseX).margin(1e-12));
+    REQUIRE(first.height == Approx(expected.height).margin(1e-12));
+    REQUIRE(first.thickness == Approx(expected.thickness).margin(1e-12));
+    REQUIRE(first.hue == expected.hue);
+    REQUIRE(first.swayPhaseOffset == Approx(expected.sway).margin(1e-12));
+    REQUIRE(first.stiffness == Approx(expected.stiffness).margin(1e-12));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/entity_skeleton_tests.cpp

@@ -0,0 +1,113 @@
+// entity_skeleton_tests.cpp
+//
+// Entity subsystem skeleton tests (architecture.md §13.2).
+//
+// Coverage:
+//   * EntityKind discriminants match the spec ({None=0, Tumbleweed=1,
+//     Snowflake=2}).
+//   * MAX_ENTITIES_PER_MONITOR is the locked cap (= 64).
+//   * sim_init defaults sim.entities to empty, capacity >= cap.
+//   * sim_set_scene clears entities (currently a no-op since the Grass
+//     scene generates none; §14/§15 add per-scene generators).
+//   * sim_tick_entities is safe on empty (no exceptions, no growth).
+//   * Tick on empty entities does not perturb other sim state (blades
+//     untouched, ambient PRNG untouched).
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+using namespace desktopgrass;
+
+TEST_CASE("EntityKind has spec-locked discriminants", "[entities][enum]") {
+    REQUIRE(static_cast<int>(EntityKind::None)       == 0);
+    REQUIRE(static_cast<int>(EntityKind::Tumbleweed) == 1);
+    REQUIRE(static_cast<int>(EntityKind::Snowflake)  == 2);
+    REQUIRE(static_cast<int>(EntityKind::Sheep)      == 3);
+    REQUIRE(static_cast<int>(EntityKind::Cat)        == 4);
+    REQUIRE(static_cast<int>(EntityKind::Bunny)      == 6);
+    REQUIRE(static_cast<int>(EntityKind::Butterfly)  == 7);
+    REQUIRE(static_cast<int>(EntityKind::Firefly)    == 8);
+    REQUIRE(static_cast<int>(EntityKind::Bird)       == 9);
+    REQUIRE(MAX_ENTITIES_PER_MONITOR == 64);
+}
+
+TEST_CASE("sim_init reserves entities capacity", "[entities][init]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    REQUIRE(sim.entities.empty());
+    REQUIRE(sim.entities.capacity() >= static_cast<std::size_t>(MAX_ENTITIES_PER_MONITOR));
+    REQUIRE(sim.entitySeed == CANONICAL_TEST_SEED);
+}
+
+TEST_CASE("sim_set_scene clears entities", "[entities][scene]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, 1.0);
+    // Push a fake entity directly to verify scene-transition removal runs.
+    Entity fake{};
+    fake.kind = EntityKind::Tumbleweed;
+    fake.x = 100.0;
+    sim.entities.push_back(fake);
+    REQUIRE(sim.entities.size() == 1);
+
+    sim_set_scene(sim, Scene::Winter);
+    REQUIRE(sim.entities.empty());
+    REQUIRE(sim.entities.capacity() >= static_cast<std::size_t>(MAX_ENTITIES_PER_MONITOR));
+}
+
+TEST_CASE("sim_tick_entities is a no-op on empty outside Grass", "[entities][tick]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, 1.0);
+    sim.currentScene = Scene::Desert;
+    const auto bladesBefore = sim.blades.size();
+    const auto prngBefore   = sim.ambientPrng.state;
+
+    sim_tick_entities(sim, 0.016);
+    sim_tick_entities(sim, 0.5);
+
+    REQUIRE(sim.entities.empty());
+    REQUIRE(sim.blades.size() == bladesBefore);
+    REQUIRE(sim.ambientPrng.state == prngBefore);
+}
+
+TEST_CASE("sim_tick_entities advances a populated entity", "[entities][tick]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, 1.0);
+    sim.currentScene = Scene::Desert;
+    Entity e{};
+    e.kind          = EntityKind::Tumbleweed;
+    e.x             = 100.0;
+    e.y             = 50.0;
+    e.vx            = 50.0;    // DIP/sec
+    e.vy            = 0.0;
+    e.size          = 10.0;
+    e.rotation      = 0.5;
+    e.rotationSpeed = 1.0;     // rad/sec
+    e.age           = 0.0;
+    e.lifetime      = -1.0;    // infinite
+    e.seed          = 0xDEADBEEF;
+    sim.entities.push_back(e);
+
+    const double dt = 0.5;
+    sim_tick_entities(sim, dt);
+
+    REQUIRE(sim.entities.size() == 1);
+    const Entity& after = sim.entities[0];
+    REQUIRE(after.x == Approx(100.0 + 50.0 * dt));
+    REQUIRE(after.y == Approx(50.0));
+    REQUIRE(after.rotation == Approx(0.5 + 1.0 * dt));
+    REQUIRE(after.age == Approx(0.0 + dt));
+    REQUIRE(after.kind == EntityKind::Tumbleweed);
+}
+
+TEST_CASE("sim_tick calls sim_tick_entities (wiring check)", "[entities][tick]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, 1.0);
+    sim.currentScene = Scene::Desert;
+    Entity e{};
+    e.kind = EntityKind::Snowflake;
+    e.x = 0.0; e.y = 0.0;
+    e.vx = 10.0; e.vy = 20.0;
+    e.size = 2.0;
+    e.age = 0.0; e.lifetime = 100.0;
+    sim.entities.push_back(e);
+
+    sim_tick(sim, 0.1, nullptr, 0);
+    REQUIRE(sim.entities.size() == 1);
+    REQUIRE(sim.entities[0].x == Approx(1.0));   // 10 * 0.1
+    REQUIRE(sim.entities[0].y == Approx(2.0));   // 20 * 0.1
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/firefly_tests.cpp

@@ -0,0 +1,195 @@
+// firefly_tests.cpp - §17.7 ambient Firefly tests.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+#include <cmath>
+#include <vector>
+
+using namespace desktopgrass;
+
+namespace {
+constexpr double Monitor1920 = 1920.0;
+
+int count_kind(const Sim& sim, EntityKind kind) {
+    return static_cast<int>(std::count_if(sim.entities.begin(), sim.entities.end(),
+        [kind](const Entity& e) { return e.kind == kind; }));
+}
+
+Sim build_grass_sim(uint64_t seed = CANONICAL_TEST_SEED) {
+    Sim sim = sim_init(seed, Monitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Grass);
+    return sim;
+}
+
+int prng_count(Prng& side, int minCount, int maxCount) {
+    const double draw = prng_uniform(side, static_cast<double>(minCount), static_cast<double>(maxCount + 1));
+    int count = static_cast<int>(std::floor(draw));
+    if (count < minCount) count = minCount;
+    if (count > maxCount) count = maxCount;
+    return count;
+}
+} // namespace
+
+TEST_CASE("Firefly constants are pinned to spec values", "[firefly][constants]") {
+    REQUIRE(FIREFLY_COUNT_MIN == 3);
+    REQUIRE(FIREFLY_COUNT_MAX == 6);
+    REQUIRE(FIREFLY_DRIFT_SPEED_MIN == Approx(4.0));
+    REQUIRE(FIREFLY_DRIFT_SPEED_MAX == Approx(10.0));
+    REQUIRE(FIREFLY_BODY_RADIUS == Approx(1.2));
+    REQUIRE(FIREFLY_GLOW_RADIUS == Approx(5.0));
+    REQUIRE(FIREFLY_BLINK_PERIOD_MIN == Approx(1.4));
+    REQUIRE(FIREFLY_BLINK_PERIOD_MAX == Approx(2.6));
+    REQUIRE(FIREFLY_BLINK_DUTY == Approx(0.55));
+    REQUIRE(FIREFLY_BLINK_FADE == Approx(0.30));
+    REQUIRE(FIREFLY_DRIFT_FREQ_X == Approx(0.4));
+    REQUIRE(FIREFLY_DRIFT_FREQ_Y == Approx(0.6));
+    REQUIRE(FIREFLY_DRIFT_AMP_X == Approx(0.6));
+    REQUIRE(FIREFLY_DRIFT_AMP_Y == Approx(8.0));
+    REQUIRE(FIREFLY_ALTITUDE_MIN == Approx(8.0));
+    REQUIRE(FIREFLY_ALTITUDE_MAX == Approx(55.0));
+    REQUIRE(FIREFLY_BODY_COLOR == 0xFFFFEE88u);
+    REQUIRE(FIREFLY_GLOW_COLOR_RGB == 0xEEDD66u);
+    REQUIRE(FIREFLY_GLOW_ALPHA_MAX == 110);
+    REQUIRE(FIREFLY_BODY_ALPHA_MAX == 255);
+    REQUIRE(FIREFLY_PRNG_SALT == 0xF13EF1E7777ull);
+}
+
+TEST_CASE("Grass generation produces firefly count in range", "[firefly][gen]") {
+    for (uint64_t i = 0; i < 128; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i * 0x9E3779B97F4A7C15ull;
+        Sim sim = build_grass_sim(seed);
+        REQUIRE(count_kind(sim, EntityKind::Firefly) >= FIREFLY_COUNT_MIN);
+        REQUIRE(count_kind(sim, EntityKind::Firefly) <= FIREFLY_COUNT_MAX);
+    }
+}
+
+TEST_CASE("Fireflies are Grass scene only", "[firefly][scene]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, Monitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE(count_kind(sim, EntityKind::Firefly) == 0);
+    sim_set_scene(sim, Scene::Winter);
+    REQUIRE(count_kind(sim, EntityKind::Firefly) == 0);
+}
+
+TEST_CASE("Generated fireflies have speed altitude and blink period ranges", "[firefly][gen]") {
+    Sim sim = build_grass_sim();
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Firefly) continue;
+        REQUIRE(e.baseSpeed >= FIREFLY_DRIFT_SPEED_MIN);
+        REQUIRE(e.baseSpeed <  FIREFLY_DRIFT_SPEED_MAX);
+        REQUIRE(e.altitudeAnchor >= FIREFLY_ALTITUDE_MIN);
+        REQUIRE(e.altitudeAnchor <  FIREFLY_ALTITUDE_MAX);
+        REQUIRE(e.blinkPeriod >= FIREFLY_BLINK_PERIOD_MIN);
+        REQUIRE(e.blinkPeriod <  FIREFLY_BLINK_PERIOD_MAX);
+    }
+}
+
+TEST_CASE("Firefly PRNG draw order matches side stream", "[firefly][prng]") {
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ FIREFLY_PRNG_SALT);
+    Sim sim = build_grass_sim();
+
+    const int expectedCount = prng_count(side, FIREFLY_COUNT_MIN, FIREFLY_COUNT_MAX);
+    REQUIRE(count_kind(sim, EntityKind::Firefly) == expectedCount);
+
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Firefly) continue;
+        const double xFrac = prng_uniform(side, 0.0, 1.0);
+        const double yFrac = prng_uniform(side, 0.0, 1.0);
+        const uint64_t vxSign = prng_next_u64(side) & 1ull;
+        const double expectedDir = vxSign != 0ull ? 1.0 : -1.0;
+        const double expectedSpeed = prng_uniform(side, FIREFLY_DRIFT_SPEED_MIN, FIREFLY_DRIFT_SPEED_MAX);
+        const double expectedBlinkPeriod = prng_uniform(side, FIREFLY_BLINK_PERIOD_MIN, FIREFLY_BLINK_PERIOD_MAX);
+        const double expectedBlinkPhase = prng_uniform(side, 0.0, 1.0);
+        const double expectedPhaseY = prng_uniform(side, 0.0, 2.0 * 3.14159265358979323846);
+        const double expectedPhaseX = prng_uniform(side, 0.0, 2.0 * 3.14159265358979323846);
+        const double expectedAltitude = FIREFLY_ALTITUDE_MIN + yFrac * (FIREFLY_ALTITUDE_MAX - FIREFLY_ALTITUDE_MIN);
+        const double expectedVx = expectedDir * expectedSpeed * (1.0 + FIREFLY_DRIFT_AMP_X * std::sin(expectedPhaseX));
+
+        REQUIRE(e.x == Approx(xFrac * Monitor1920));
+        REQUIRE(e.altitudeAnchor == Approx(expectedAltitude));
+        REQUIRE(e.baseSpeed == Approx(expectedSpeed));
+        REQUIRE(e.blinkPeriod == Approx(expectedBlinkPeriod));
+        REQUIRE(e.blinkPhase == Approx(expectedBlinkPhase));
+        REQUIRE(e.vx == Approx(expectedVx));
+        REQUIRE(e.phaseY == Approx(expectedPhaseY));
+        REQUIRE(e.phaseX == Approx(expectedPhaseX));
+        ++seen;
+    }
+    REQUIRE(seen == expectedCount);
+}
+
+TEST_CASE("Firefly edge wrap preserves altitude anchor", "[firefly][motion]") {
+    Sim sim = build_grass_sim();
+    auto it = std::find_if(sim.entities.begin(), sim.entities.end(), [](const Entity& e) { return e.kind == EntityKind::Firefly; });
+    REQUIRE(it != sim.entities.end());
+    const double margin = FIREFLY_GLOW_RADIUS;
+    it->x = Monitor1920 + margin + 1.0;
+    it->vx = std::abs(it->vx);
+    const double altitude = it->altitudeAnchor;
+    sim.currentScene = Scene::Desert;
+
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(it->x == Approx(-margin));
+    REQUIRE(it->altitudeAnchor == Approx(altitude));
+}
+
+TEST_CASE("Fireflies do not interact with cuts or pets", "[firefly][interaction]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, Monitor1920, DEFAULT_DENSITY);
+    sim.entities.clear();
+    Entity firefly{};
+    firefly.kind = EntityKind::Firefly;
+    firefly.x = 500.0;
+    firefly.y = sim.windowHeight - STRIP_HEIGHT - 5.0;
+    firefly.vx = FIREFLY_DRIFT_SPEED_MIN;
+    firefly.baseSpeed = FIREFLY_DRIFT_SPEED_MIN;
+    firefly.altitudeAnchor = FIREFLY_ALTITUDE_MIN;
+    firefly.blinkPeriod = FIREFLY_BLINK_PERIOD_MIN;
+    firefly.lifetime = -1.0;
+    sim.entities.push_back(firefly);
+    Entity sheep{};
+    sheep.kind = EntityKind::Sheep;
+    sheep.x = firefly.x;
+    sheep.y = sim.windowHeight - SHEEP_BODY_HEIGHT - SHEEP_LEG_LENGTH;
+    sheep.vx = SHEEP_WALK_SPEED_MIN;
+    sheep.state = SHEEP_STATE_WALKING;
+    sheep.stateTimer = 10.0;
+    sim.entities.push_back(sheep);
+
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x = firefly.x;
+    ev.y = firefly.y;
+    sim_apply_click(sim, ev);
+
+    REQUIRE(sim.entities[0].kind == EntityKind::Firefly);
+    REQUIRE(sim.entities[0].baseSpeed == Approx(FIREFLY_DRIFT_SPEED_MIN));
+    REQUIRE(sim.entities[1].state == SHEEP_STATE_WALKING);
+    for (const Blade& b : sim.blades) REQUIRE(b.cutAnimStart < 0.0);
+}
+
+TEST_CASE("Firefly blink brightness has on and off phases", "[firefly][blink]") {
+    const double period = 2.0;
+    REQUIRE(firefly_blink_brightness(period * 0.25, period, 0.0) == Approx(1.0));
+    REQUIRE(firefly_blink_brightness(period * 0.80, period, 0.0) == Approx(0.0));
+}
+
+TEST_CASE("Firefly phases decorrelate visible brightness", "[firefly][blink]") {
+    const double period = 2.0;
+    const double phases[] = { 0.0, 0.0375, 0.075, 0.1125, 0.25, 0.80 };
+    std::vector<double> distinct;
+    for (double phase : phases) {
+        const double b = firefly_blink_brightness(0.0, period, phase);
+        bool seen = false;
+        for (double existing : distinct) {
+            if (std::fabs(existing - b) < 1e-6) { seen = true; break; }
+        }
+        if (!seen) distinct.push_back(b);
+    }
+    REQUIRE(distinct.size() >= 4);
+}
+

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/flower_tests.cpp

@@ -0,0 +1,54 @@
+// flower_tests.cpp
+//
+// Tests for §5 flower stream + §7 head-render contract.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <cmath>
+#include <vector>
+
+using namespace desktopgrass;
+
+TEST_CASE("flower stream is deterministic for a given seed", "[flowers]") {
+    std::vector<Blade> a, b;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, a);
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, b);
+    REQUIRE(a.size() == b.size());
+    for (std::size_t i = 0; i < a.size(); ++i) {
+        REQUIRE(a[i].isFlower           == b[i].isFlower);
+        REQUIRE(a[i].flowerHeadColorIdx == b[i].flowerHeadColorIdx);
+        REQUIRE(a[i].flowerHeadRadius   == b[i].flowerHeadRadius);
+        REQUIRE(a[i].heightBonus        == b[i].heightBonus);
+    }
+}
+
+TEST_CASE("flower count is within 3-sigma of FLOWER_PROBABILITY", "[flowers]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    REQUIRE(blades.size() > 100);
+
+    std::size_t flowerCount = 0;
+    for (const Blade& b : blades) if (b.isFlower) ++flowerCount;
+
+    const double n   = static_cast<double>(blades.size());
+    const double p   = FLOWER_PROBABILITY;
+    const double mu  = n * p;
+    const double sd  = std::sqrt(n * p * (1.0 - p));
+    // 3-sigma tolerance keeps this test stable across spec-conformant
+    // PRNG sequences. For seed=0x6B6173746F, n=321 we expect ~12.84 with
+    // sd≈3.51, so [2,24] is the acceptable range.
+    REQUIRE(flowerCount >= static_cast<std::size_t>(std::floor(mu - 3.0 * sd)));
+    REQUIRE(flowerCount <= static_cast<std::size_t>(std::ceil(mu + 3.0 * sd)));
+}
+
+TEST_CASE("flower stream does not perturb the main stream", "[flowers][conformance]") {
+    // Regenerate blades and assert the main-stream fields match the
+    // canonical snapshot. This is implicitly covered by blade_gen_tests
+    // (the first/last 10 still match), but pin it here for clarity.
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    REQUIRE(blades.size() > 0);
+    REQUIRE(blades[0].baseX == Approx(4.941073726820111).margin(1e-12));
+    REQUIRE(blades[0].height == Approx(24.469991818248864).margin(1e-12));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/gust_tests.cpp

@@ -0,0 +1,164 @@
+// gust_tests.cpp
+//
+// Gust impulse model tests (architecture.md §8).
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <cmath>
+
+using namespace desktopgrass;
+
+namespace {
+
+Sim make_sim_with_blades(std::initializer_list<double> baseXs) {
+    Sim sim;
+    sim.windowHeight = STRIP_HEIGHT + HEADROOM;
+    for (double x : baseXs) {
+        Blade b{};
+        b.baseX            = x;
+        b.height           = 20.0;
+        b.thickness        = 1.5;
+        b.swayPhaseOffset  = 0.0;
+        b.stiffness        = 1.0;
+        b.cutHeight        = 1.0;
+        b.cutInitialHeight = 1.0;
+        b.cutAnimStart     = -1.0;
+        sim.blades.push_back(b);
+    }
+    return sim;
+}
+
+InputEvent move(double x, double y, double t) {
+    return InputEvent{ EventType::Move, x, y, t };
+}
+
+} // anonymous
+
+TEST_CASE("first move event is a baseline; no impulse", "[gust]") {
+    Sim sim = make_sim_with_blades({100.0});
+    const double groundY = sim.windowHeight;
+    const double bandY   = groundY - 10.0; // in band
+
+    sim_apply_move(sim, move(100.0, bandY, 0.0));
+    REQUIRE(sim.blades[0].gustVelocity == Approx(0.0));
+    REQUIRE(sim.prevCursorTime == Approx(0.0));
+}
+
+TEST_CASE("a second move inside the band emits an impulse", "[gust]") {
+    Sim sim = make_sim_with_blades({100.0, 200.0, 400.0});
+    const double bandY = sim.windowHeight - 10.0;
+
+    sim_apply_move(sim, move(  0.0, bandY, 0.0));
+    sim_apply_move(sim, move(100.0, bandY, 0.05));  // velocity = 2000 DIP/sec
+
+    // Blade at 100 is right under the cursor → max impulse.
+    // Expected magnitude:
+    //   capped = 2000 (≤ 4000 cap)
+    //   impulseMagnitude = 2000 * 0.003 = 6.0
+    //   smoothstep at distance 0 = 1.0
+    //   smoothstep at distance 100/150 (blade @ 200) = (1-2/3)² * (3 - 2*(1-2/3))
+    REQUIRE(sim.blades[0].gustVelocity == Approx(6.0).margin(1e-9));
+    // Blade outside radius (400, dist=300 > 150) → no impulse.
+    REQUIRE(sim.blades[2].gustVelocity == Approx(0.0));
+    REQUIRE(sim.blades[1].gustVelocity > 0.0);
+    REQUIRE(sim.blades[1].gustVelocity < 6.0);
+}
+
+TEST_CASE("impulse is signed by motion direction", "[gust]") {
+    Sim left  = make_sim_with_blades({100.0});
+    Sim right = make_sim_with_blades({100.0});
+
+    const double y = left.windowHeight - 10.0;
+    sim_apply_move(left,  move(200.0, y, 0.0));
+    sim_apply_move(left,  move(100.0, y, 0.05)); // moving left
+
+    sim_apply_move(right, move(  0.0, y, 0.0));
+    sim_apply_move(right, move(100.0, y, 0.05)); // moving right
+
+    REQUIRE(left.blades[0].gustVelocity  < 0.0);
+    REQUIRE(right.blades[0].gustVelocity > 0.0);
+    REQUIRE(std::fabs(left.blades[0].gustVelocity) ==
+            Approx(std::fabs(right.blades[0].gustVelocity)).margin(1e-9));
+}
+
+TEST_CASE("cursor speed is capped at MAX_CURSOR_SPEED", "[gust]") {
+    Sim sim = make_sim_with_blades({100.0});
+    const double y = sim.windowHeight - 10.0;
+
+    sim_apply_move(sim, move(0.0,      y, 0.0));
+    sim_apply_move(sim, move(100000.0, y, 0.05)); // velocity ≈ 2e6 DIP/sec
+
+    // capped magnitude = MAX_CURSOR_SPEED * IMPULSE_SCALE = 4000 * 0.003 = 12
+    // but the blade is at distance ~100k from cursor: outside radius → no impulse
+    REQUIRE(sim.blades[0].gustVelocity == Approx(0.0));
+}
+
+TEST_CASE("max impulse at the cursor equals capped magnitude", "[gust]") {
+    Sim sim = make_sim_with_blades({1000.0});
+    const double y = sim.windowHeight - 10.0;
+
+    sim_apply_move(sim, move(0.0,    y, 0.0));
+    sim_apply_move(sim, move(1000.0, y, 0.0001)); // velocity huge → saturates
+
+    // Saturated: cursor lands at x=1000 (blade), distance=0, smoothstep=1.0
+    REQUIRE(sim.blades[0].gustVelocity ==
+            Approx(MAX_CURSOR_SPEED * IMPULSE_SCALE).margin(1e-9));
+}
+
+TEST_CASE("moves outside the gust band don't emit impulses", "[gust]") {
+    Sim sim = make_sim_with_blades({100.0});
+    const double y_above_band = sim.windowHeight - STRIP_HEIGHT - HEADROOM - 20.0;
+
+    sim_apply_move(sim, move(  0.0, y_above_band, 0.0));
+    sim_apply_move(sim, move(100.0, y_above_band, 0.05));
+    REQUIRE(sim.blades[0].gustVelocity == Approx(0.0));
+}
+
+TEST_CASE("out-of-band move updates baseline; re-entry parity", "[gust]") {
+    Sim sim = make_sim_with_blades({700.0});
+    const double inBandY    = sim.windowHeight - 10.0;
+    const double outOfBandY = sim.windowHeight - STRIP_HEIGHT - HEADROOM - 20.0;
+
+    // t0 in-band: primes baseline (first event, no impulse).
+    sim_apply_move(sim, move(500.0, inBandY, 0.0));
+    // t1 out-of-band: updates baseline but emits no impulse.
+    sim_apply_move(sim, move(520.0, outOfBandY, 0.05));
+    REQUIRE(sim.prevCursorX    == Approx(520.0));
+    REQUIRE(sim.prevCursorTime == Approx(0.05));
+    REQUIRE(sim.blades[0].gustVelocity == Approx(0.0));
+
+    // t2 re-enter in-band: emits impulse off the out-of-band baseline.
+    sim_apply_move(sim, move(700.0, inBandY, 0.10));
+
+    const double dtEv  = std::max(0.10 - 0.05, 1.0 / 1000.0);
+    const double velX  = (700.0 - 520.0) / dtEv;
+    const double capped = std::max(-MAX_CURSOR_SPEED, std::min(velX, MAX_CURSOR_SPEED));
+    const double expected = capped * IMPULSE_SCALE; // distance 0 → smoothstep = 1
+    REQUIRE(sim.blades[0].gustVelocity == Approx(expected).margin(1e-9));
+}
+
+TEST_CASE("large time gap resets cursor baseline", "[gust]") {
+    Sim sim = make_sim_with_blades({100.0});
+    const double y = sim.windowHeight - 10.0;
+
+    sim_apply_move(sim, move(  0.0, y, 0.0));
+    sim_apply_move(sim, move(500.0, y, 0.5));   // > CURSOR_REINIT_GAP_SEC (0.25)
+    REQUIRE(sim.blades[0].gustVelocity == Approx(0.0));
+}
+
+TEST_CASE("impulse falls off smoothly with distance", "[gust]") {
+    Sim sim = make_sim_with_blades({100.0, 130.0, 175.0, 200.0, 249.0, 251.0});
+    const double y = sim.windowHeight - 10.0;
+
+    sim_apply_move(sim, move(  0.0, y, 0.0));
+    sim_apply_move(sim, move(100.0, y, 0.05));
+
+    // Monotonic falloff: cursor at 100.
+    REQUIRE(sim.blades[0].gustVelocity > sim.blades[1].gustVelocity);
+    REQUIRE(sim.blades[1].gustVelocity > sim.blades[2].gustVelocity);
+    REQUIRE(sim.blades[2].gustVelocity > sim.blades[3].gustVelocity);
+    REQUIRE(sim.blades[3].gustVelocity > sim.blades[4].gustVelocity);
+    // Just outside radius (251 → distance 151 > 150) → zero.
+    REQUIRE(sim.blades[5].gustVelocity == Approx(0.0));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/hedgehog_tests.cpp

@@ -0,0 +1,371 @@
+// hedgehog_tests.cpp
+//
+// §17.9 Hedgehog critter tests. Mirrors Win2D HedgehogTests.cs.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cwchar>
+
+using namespace desktopgrass;
+
+namespace {
+
+constexpr double Monitor1920 = 1920.0;
+
+int count_kind(const Sim& sim, EntityKind kind) {
+    return static_cast<int>(std::count_if(sim.entities.begin(), sim.entities.end(),
+        [kind](const Entity& e) { return e.kind == kind; }));
+}
+
+Sim build_sim(uint64_t seed = CANONICAL_TEST_SEED) {
+    return sim_init(seed, Monitor1920, DEFAULT_DENSITY);
+}
+
+Sim build_grass_sim(uint64_t seed = CANONICAL_TEST_SEED) {
+    Sim sim = build_sim(seed);
+    sim_set_scene(sim, Scene::Grass);
+    sim_set_critter(sim, CritterKind::Bunny);
+    return sim;
+}
+
+Entity hedgehog_entity(double x = 500.0, double vx = HEDGEHOG_WALK_SPEED_MIN) {
+    Entity e{};
+    e.kind = EntityKind::Hedgehog;
+    e.size = HEDGEHOG_BODY_RADIUS;
+    e.x = x;
+    e.y = STRIP_HEIGHT + HEADROOM - HEDGEHOG_BODY_HEIGHT - HEDGEHOG_LEG_LENGTH;
+    e.vx = vx;
+    e.vy = 0.0;
+    e.rotationSpeed = std::abs(vx);
+    e.lifetime = -1.0;
+    e.state = HEDGEHOG_STATE_WALKING;
+    e.stateTimer = HEDGEHOG_WALK_DURATION_MIN;
+    e.previousState = HEDGEHOG_STATE_WALKING;
+    return e;
+}
+
+InputEvent click_event(double x, double y) {
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x = x;
+    ev.y = y;
+    ev.time = 0.0;
+    return ev;
+}
+
+int prng_count(Prng& side, int minCount, int maxCount) {
+    const double draw = prng_uniform(side, static_cast<double>(minCount), static_cast<double>(maxCount + 1));
+    int count = static_cast<int>(std::floor(draw));
+    if (count < minCount) count = minCount;
+    if (count > maxCount) count = maxCount;
+    return count;
+}
+
+void advance_sheep(Prng& side, int count) {
+    for (int i = 0; i < count; ++i) {
+        const double margin = SHEEP_BODY_RADIUS + 8.0;
+        (void)prng_uniform(side, margin, Monitor1920 - margin);
+        (void)prng_uniform(side, SHEEP_WALK_SPEED_MIN, SHEEP_WALK_SPEED_MAX);
+        (void)prng_uniform(side, 0.0, 1.0);
+        (void)prng_next_u32(side);
+        (void)prng_uniform(side, SHEEP_WALK_DURATION_MIN, SHEEP_WALK_DURATION_MAX);
+        (void)prng_index(side, static_cast<uint32_t>(sizeof(SHEEP_NAME_POOL) / sizeof(SHEEP_NAME_POOL[0])));
+    }
+}
+
+void advance_cats(Prng& side, int count) {
+    for (int i = 0; i < count; ++i) {
+        const double margin = CAT_BODY_RADIUS + 8.0;
+        (void)prng_uniform(side, margin, Monitor1920 - margin);
+        (void)prng_uniform(side, CAT_WALK_SPEED_MIN, CAT_WALK_SPEED_MAX);
+        (void)prng_uniform(side, 0.0, 1.0);
+        (void)prng_next_u32(side);
+        (void)prng_uniform(side, CAT_WALK_DURATION_MIN, CAT_WALK_DURATION_MAX);
+        (void)prng_index(side, static_cast<uint32_t>(sizeof(CAT_NAME_POOL) / sizeof(CAT_NAME_POOL[0])));
+        (void)prng_index(side, static_cast<uint32_t>(CAT_COAT_VARIANT_COUNT));
+    }
+}
+
+void advance_bunnies(Prng& side, int count) {
+    for (int i = 0; i < count; ++i) {
+        (void)prng_uniform(side, 0.0, 1.0);
+        (void)prng_next_u64(side);
+        (void)prng_uniform(side, BUNNY_HOP_SPEED_MIN, BUNNY_HOP_SPEED_MAX);
+        (void)prng_index(side, static_cast<uint32_t>(sizeof(BUNNY_NAME_POOL) / sizeof(BUNNY_NAME_POOL[0])));
+    }
+}
+
+bool hedgehog_name_in_pool(const Entity& e) {
+    if (e.nameIndex >= sizeof(HEDGEHOG_NAME_POOL) / sizeof(HEDGEHOG_NAME_POOL[0])) return false;
+    const wchar_t* name = HEDGEHOG_NAME_POOL[e.nameIndex];
+    for (const wchar_t* candidate : HEDGEHOG_NAME_POOL) {
+        if (std::wcscmp(name, candidate) == 0) return true;
+    }
+    return false;
+}
+
+} // namespace
+
+TEST_CASE("Hedgehog constants are pinned to spec values", "[hedgehog][constants]") {
+    REQUIRE(HEDGEHOG_COUNT_MIN == 0);
+    REQUIRE(HEDGEHOG_COUNT_MAX == 1);
+    REQUIRE(HEDGEHOG_COUNT_PROBABILITY == Approx(0.55));
+    REQUIRE(HEDGEHOG_WALK_SPEED_MIN == Approx(4.0));
+    REQUIRE(HEDGEHOG_WALK_SPEED_MAX == Approx(8.0));
+    REQUIRE(HEDGEHOG_BODY_RADIUS == Approx(9.0));
+    REQUIRE(HEDGEHOG_BODY_HEIGHT == Approx(5.5));
+    REQUIRE(HEDGEHOG_HEAD_RADIUS == Approx(3.6));
+    REQUIRE(HEDGEHOG_NOSE_RADIUS == Approx(0.8));
+    REQUIRE(HEDGEHOG_LEG_LENGTH == Approx(2.5));
+    REQUIRE(HEDGEHOG_SPIKE_COUNT == 14);
+    REQUIRE(HEDGEHOG_SPIKE_LENGTH == Approx(3.0));
+    REQUIRE(HEDGEHOG_SPIKE_WIDTH == Approx(1.4));
+    REQUIRE(HEDGEHOG_SPIKE_ARC_START_DEG == Approx(-20.0));
+    REQUIRE(HEDGEHOG_SPIKE_ARC_END_DEG == Approx(200.0));
+    REQUIRE(HEDGEHOG_BODY_COLOR == 0xFF5C4633u);
+    REQUIRE(HEDGEHOG_SPIKE_COLOR == 0xFF3A2A1Fu);
+    REQUIRE(HEDGEHOG_SPIKE_TIP_COLOR == 0xFF1E150Eu);
+    REQUIRE(HEDGEHOG_NOSE_COLOR == 0xFF1A1208u);
+    REQUIRE(HEDGEHOG_EYE_COLOR == 0xFF1A1208u);
+    REQUIRE(HEDGEHOG_STATE_WALKING == 0);
+    REQUIRE(HEDGEHOG_STATE_SNUFFLING == 1);
+    REQUIRE(HEDGEHOG_STATE_IDLE == 2);
+    REQUIRE(HEDGEHOG_STATE_SLEEPING == 3);
+    REQUIRE(HEDGEHOG_STATE_CURLED == 4);
+    REQUIRE(HEDGEHOG_WALK_DURATION_MIN == Approx(6.0));
+    REQUIRE(HEDGEHOG_WALK_DURATION_MAX == Approx(12.0));
+    REQUIRE(HEDGEHOG_SNUFFLE_DURATION_MIN == Approx(3.0));
+    REQUIRE(HEDGEHOG_SNUFFLE_DURATION_MAX == Approx(6.0));
+    REQUIRE(HEDGEHOG_IDLE_DURATION_MIN == Approx(1.5));
+    REQUIRE(HEDGEHOG_IDLE_DURATION_MAX == Approx(3.0));
+    REQUIRE(HEDGEHOG_SLEEP_DURATION_MIN == Approx(10.0));
+    REQUIRE(HEDGEHOG_SLEEP_DURATION_MAX == Approx(25.0));
+    REQUIRE(HEDGEHOG_CURL_DURATION_MIN == Approx(3.0));
+    REQUIRE(HEDGEHOG_CURL_DURATION_MAX == Approx(5.5));
+    REQUIRE(HEDGEHOG_SNUFFLE_PROBABILITY == Approx(0.55));
+    REQUIRE(HEDGEHOG_IDLE_PROBABILITY == Approx(0.30));
+    REQUIRE(HEDGEHOG_SLEEP_PROB == Approx(0.50));
+    REQUIRE(HEDGEHOG_STARTLE_RADIUS == Approx(70.0));
+    REQUIRE(HEDGEHOG_SNUFFLE_HEAD_FREQ == Approx(5.0));
+    REQUIRE(HEDGEHOG_SNUFFLE_HEAD_AMP == Approx(0.7));
+    REQUIRE(HEDGEHOG_WADDLE_FREQ == Approx(4.0));
+    REQUIRE(HEDGEHOG_WADDLE_AMP == Approx(0.8));
+    REQUIRE(HEDGEHOG_ZZZ_CYCLE_SEC == Approx(SHEEP_ZZZ_CYCLE_SEC));
+    REQUIRE(HEDGEHOG_ZZZ_RISE == Approx(SHEEP_ZZZ_RISE * 0.5));
+    REQUIRE(HEDGEHOG_ZZZ_SIZE_START == Approx(SHEEP_ZZZ_SIZE_START * 0.6));
+    REQUIRE(HEDGEHOG_ZZZ_SIZE_END == Approx(SHEEP_ZZZ_SIZE_END * 0.6));
+    REQUIRE(sizeof(HEDGEHOG_NAME_POOL) / sizeof(HEDGEHOG_NAME_POOL[0]) == 12);
+    REQUIRE(std::wcscmp(HEDGEHOG_NAME_POOL[0], L"Bristle") == 0);
+    REQUIRE(std::wcscmp(HEDGEHOG_NAME_POOL[11], L"Burdock") == 0);
+}
+
+TEST_CASE("Hedgehog count distribution is probabilistic rare sighting", "[hedgehog][gen]") {
+    constexpr int N = 1000;
+    int present = 0;
+    for (uint64_t i = 0; i < N; ++i) {
+        const uint64_t seed = CANONICAL_TEST_SEED + i * 0x9E3779B97F4A7C15ull;
+        Sim sim = build_grass_sim(seed);
+        const int count = count_kind(sim, EntityKind::Hedgehog);
+        REQUIRE(count >= HEDGEHOG_COUNT_MIN);
+        REQUIRE(count <= HEDGEHOG_COUNT_MAX);
+        present += count;
+    }
+    REQUIRE(static_cast<double>(present) / N == Approx(HEDGEHOG_COUNT_PROBABILITY).margin(0.05));
+}
+
+TEST_CASE("Hedgehogs are Grass scene only", "[hedgehog][scene]") {
+    Sim sim = build_sim();
+    sim_set_scene(sim, Scene::Desert);
+    REQUIRE(count_kind(sim, EntityKind::Hedgehog) == 0);
+    sim_set_scene(sim, Scene::Winter);
+    REQUIRE(count_kind(sim, EntityKind::Hedgehog) == 0);
+}
+
+TEST_CASE("Generated hedgehogs have speed range", "[hedgehog][gen]") {
+    bool sawHedgehog = false;
+    for (uint64_t i = 0; i < 128; ++i) {
+        Sim sim = build_grass_sim(CANONICAL_TEST_SEED + i * 0xD1B54A32D192ED03ull);
+        for (const Entity& e : sim.entities) {
+            if (e.kind != EntityKind::Hedgehog) continue;
+            sawHedgehog = true;
+            REQUIRE(std::abs(e.vx) >= HEDGEHOG_WALK_SPEED_MIN);
+            REQUIRE(std::abs(e.vx) <= HEDGEHOG_WALK_SPEED_MAX);
+            REQUIRE(e.rotationSpeed == Approx(std::abs(e.vx)));
+        }
+    }
+    REQUIRE(sawHedgehog);
+}
+
+TEST_CASE("Generated hedgehogs have names in pool", "[hedgehog][gen]") {
+    bool sawHedgehog = false;
+    for (uint64_t i = 0; i < 128; ++i) {
+        Sim sim = build_grass_sim(CANONICAL_TEST_SEED + i * 0x94D049BB133111EBull);
+        for (const Entity& e : sim.entities) {
+            if (e.kind != EntityKind::Hedgehog) continue;
+            sawHedgehog = true;
+            REQUIRE(hedgehog_name_in_pool(e));
+        }
+    }
+    REQUIRE(sawHedgehog);
+}
+
+TEST_CASE("Hedgehog PRNG draw order follows sheep cats and bunnies", "[hedgehog][prng]") {
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+    Sim sim = build_grass_sim();
+
+    const int sheepCount = prng_count(side, SHEEP_COUNT_MIN, SHEEP_COUNT_MAX);
+    advance_sheep(side, sheepCount);
+    const int catCount = prng_count(side, CAT_COUNT_MIN, CAT_COUNT_MAX);
+    advance_cats(side, catCount);
+    const int bunnyCount = prng_count(side, BUNNY_COUNT_MIN, BUNNY_COUNT_MAX);
+    advance_bunnies(side, bunnyCount);
+
+    const double hasDraw = prng_uniform(side, 0.0, 1.0);
+    const int hedgehogCount = hasDraw < HEDGEHOG_COUNT_PROBABILITY ? 1 : 0;
+    REQUIRE(count_kind(sim, EntityKind::Hedgehog) == hedgehogCount);
+
+    int seen = 0;
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::Hedgehog) continue;
+        const double margin = HEDGEHOG_BODY_RADIUS + 8.0;
+        const double xFrac = prng_uniform(side, 0.0, 1.0);
+        const double expectedX = margin + xFrac * (Monitor1920 - 2.0 * margin);
+        const uint64_t vxSign = prng_next_u64(side) & 1ull;
+        const double expectedDir = vxSign != 0ull ? 1.0 : -1.0;
+        const double expectedSpeed = prng_uniform(side, HEDGEHOG_WALK_SPEED_MIN, HEDGEHOG_WALK_SPEED_MAX);
+        const uint8_t expectedName = static_cast<uint8_t>(prng_index(side,
+            static_cast<uint32_t>(sizeof(HEDGEHOG_NAME_POOL) / sizeof(HEDGEHOG_NAME_POOL[0]))));
+        REQUIRE(e.x == Approx(expectedX));
+        REQUIRE(e.vx == Approx(expectedDir * expectedSpeed));
+        REQUIRE(e.nameIndex == expectedName);
+        ++seen;
+    }
+    REQUIRE(seen == hedgehogCount);
+}
+
+TEST_CASE("Hedgehog edge bounce flips direction", "[hedgehog][motion]") {
+    Sim sim = build_sim();
+    sim.currentScene = Scene::Desert;
+    sim.entities.clear();
+    Entity e = hedgehog_entity(Monitor1920 - (HEDGEHOG_BODY_RADIUS + 2.0) + 0.1, HEDGEHOG_WALK_SPEED_MIN);
+    e.stateTimer = 10.0;
+    sim.entities.push_back(e);
+
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(sim.entities.front().vx < 0.0);
+}
+
+TEST_CASE("Hedgehog startle radius curls without flipping vx", "[hedgehog][click]") {
+    Sim sim = build_sim();
+    sim.entities.clear();
+    Entity e = hedgehog_entity(500.0, -HEDGEHOG_WALK_SPEED_MIN);
+    e.state = HEDGEHOG_STATE_WALKING;
+    e.stateTimer = 10.0;
+    sim.entities.push_back(e);
+
+    sim_apply_click(sim, click_event(e.x + 10.0, e.y));
+    REQUIRE(sim.entities.front().state == HEDGEHOG_STATE_CURLED);
+    REQUIRE(sim.entities.front().vx == Approx(-HEDGEHOG_WALK_SPEED_MIN));
+    REQUIRE(sim.entities.front().stateTimer >= HEDGEHOG_CURL_DURATION_MIN);
+    REQUIRE(sim.entities.front().stateTimer <= HEDGEHOG_CURL_DURATION_MAX);
+
+    Sim outside = build_sim();
+    outside.entities.clear();
+    Entity far = hedgehog_entity(500.0, HEDGEHOG_WALK_SPEED_MIN);
+    outside.entities.push_back(far);
+    sim_apply_click(outside, click_event(far.x + HEDGEHOG_STARTLE_RADIUS + 10.0, far.y));
+    REQUIRE(outside.entities.front().state == HEDGEHOG_STATE_WALKING);
+    REQUIRE(outside.entities.front().vx == Approx(HEDGEHOG_WALK_SPEED_MIN));
+}
+
+TEST_CASE("Hedgehog curl auto uncurls to previous state", "[hedgehog][state]") {
+    Sim sim = build_sim();
+    sim.entities.clear();
+    Entity e = hedgehog_entity(500.0, HEDGEHOG_WALK_SPEED_MIN);
+    e.state = HEDGEHOG_STATE_IDLE;
+    e.stateTimer = 2.5;
+    sim.entities.push_back(e);
+
+    sim_apply_click(sim, click_event(e.x, e.y));
+    REQUIRE(sim.entities.front().state == HEDGEHOG_STATE_CURLED);
+    sim_tick_entities(sim, HEDGEHOG_CURL_DURATION_MAX + 0.1);
+
+    REQUIRE(sim.entities.front().state == HEDGEHOG_STATE_IDLE);
+    REQUIRE(sim.entities.front().vx == Approx(HEDGEHOG_WALK_SPEED_MIN));
+}
+
+TEST_CASE("Hedgehog wakes from sleep on startle and does not resume sleep", "[hedgehog][click]") {
+    Sim sim = build_sim();
+    sim.entities.clear();
+    Entity e = hedgehog_entity(500.0, HEDGEHOG_WALK_SPEED_MIN);
+    e.state = HEDGEHOG_STATE_SLEEPING;
+    e.stateTimer = 10.0;
+    sim.entities.push_back(e);
+
+    sim_apply_click(sim, click_event(e.x + 10.0, e.y));
+    REQUIRE(sim.entities.front().state == HEDGEHOG_STATE_CURLED);
+    REQUIRE(sim.entities.front().state != HEDGEHOG_STATE_SLEEPING);
+    sim_tick_entities(sim, HEDGEHOG_CURL_DURATION_MAX + 0.1);
+
+    REQUIRE(sim.entities.front().state == HEDGEHOG_STATE_WALKING);
+    REQUIRE(sim.entities.front().state != HEDGEHOG_STATE_SLEEPING);
+}
+
+TEST_CASE("Hedgehog state transition probabilities are stable", "[hedgehog][state]") {
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+    constexpr int N = 10000;
+    int snuffle = 0;
+    int idle = 0;
+    int sleep = 0;
+    for (int i = 0; i < N; ++i) {
+        const uint8_t state = hedgehog_choose_rest_state(p);
+        if (state == HEDGEHOG_STATE_SNUFFLING) ++snuffle;
+        else if (state == HEDGEHOG_STATE_IDLE) ++idle;
+        else if (state == HEDGEHOG_STATE_SLEEPING) ++sleep;
+    }
+
+    const double sleepProb = HEDGEHOG_SLEEP_PROB;
+    const double activeWeight = HEDGEHOG_SNUFFLE_PROBABILITY + HEDGEHOG_IDLE_PROBABILITY;
+    const double expectedSnuffle = (1.0 - sleepProb) * HEDGEHOG_SNUFFLE_PROBABILITY / activeWeight;
+    const double expectedIdle = (1.0 - sleepProb) * HEDGEHOG_IDLE_PROBABILITY / activeWeight;
+    REQUIRE(static_cast<double>(sleep) / N == Approx(sleepProb).margin(0.02));
+    REQUIRE(static_cast<double>(snuffle) / N == Approx(expectedSnuffle).margin(0.02));
+    REQUIRE(static_cast<double>(idle) / N == Approx(expectedIdle).margin(0.02));
+}
+
+TEST_CASE("Hedgehog sleep probability is stable", "[hedgehog][state]") {
+    constexpr int N = 20000;
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED ^ 0x1234ull);
+    int sleep = 0;
+    for (int i = 0; i < N; ++i) {
+        if (hedgehog_choose_rest_state(p) == HEDGEHOG_STATE_SLEEPING) ++sleep;
+    }
+    REQUIRE(static_cast<double>(sleep) / N == Approx(HEDGEHOG_SLEEP_PROB).margin(0.02));
+}
+
+TEST_CASE("Hedgehog has no active interaction states", "[hedgehog][state]") {
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED ^ 0xCAFEull);
+    for (int i = 0; i < 1000; ++i) {
+        const uint8_t state = hedgehog_choose_rest_state(p);
+        REQUIRE((state == HEDGEHOG_STATE_SNUFFLING
+              || state == HEDGEHOG_STATE_IDLE
+              || state == HEDGEHOG_STATE_SLEEPING));
+    }
+
+    Sim sim = build_sim();
+    sim.entities.clear();
+    Entity e = hedgehog_entity(500.0, HEDGEHOG_WALK_SPEED_MIN);
+    e.stateTimer = 10.0;
+    sim.entities.push_back(e);
+    sim_tick_entities(sim, 0.016);
+    REQUIRE(sim.entities.front().state == HEDGEHOG_STATE_WALKING);
+    REQUIRE(std::abs(sim.entities.front().vx) == Approx(HEDGEHOG_WALK_SPEED_MIN));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/main.cpp

@@ -0,0 +1,7 @@
+// main.cpp
+//
+// Catch2 entry point. The Sim.cpp translation unit is also linked in via the
+// vcxproj's source list so we can test it directly.
+
+#define CATCH_CONFIG_MAIN
+#include "../third_party/catch2/catch.hpp"

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/mushroom_tests.cpp

@@ -0,0 +1,89 @@
+// mushroom_tests.cpp
+//
+// Tests for §5 mushroom stream + §7 mushroom-render contract.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+#include <cmath>
+#include <vector>
+
+using namespace desktopgrass;
+
+TEST_CASE("mushroom stream is deterministic for a given seed", "[mushrooms]") {
+    std::vector<Blade> a, b;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, a);
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, b);
+    REQUIRE(a.size() == b.size());
+    for (std::size_t i = 0; i < a.size(); ++i) {
+        REQUIRE(a[i].isMushroom            == b[i].isMushroom);
+        REQUIRE(a[i].mushroomCapColorIdx   == b[i].mushroomCapColorIdx);
+        REQUIRE(a[i].mushroomCapWidth      == b[i].mushroomCapWidth);
+        REQUIRE(a[i].mushroomCapHeight     == b[i].mushroomCapHeight);
+        REQUIRE(a[i].mushroomStemHeight    == b[i].mushroomStemHeight);
+        REQUIRE(a[i].mushroomStemThickness == b[i].mushroomStemThickness);
+    }
+}
+
+TEST_CASE("mushroom count is within 3-sigma of MUSHROOM_PROBABILITY", "[mushrooms]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    REQUIRE(blades.size() > 100);
+
+    std::size_t mushroomCount = 0;
+    for (const Blade& b : blades) if (b.isMushroom) ++mushroomCount;
+
+    const double n   = static_cast<double>(blades.size());
+    const double p   = MUSHROOM_PROBABILITY;
+    const double mu  = n * p;
+    const double sd  = std::sqrt(n * p * (1.0 - p));
+    // 3-sigma tolerance keeps this test stable across spec-conformant
+    // PRNG sequences. For seed=0x6B6173746F, n=321 we expect ~8.03 with
+    // sd≈2.80, so the inclusive 3-sigma range is roughly [0, 17].
+    const double lo  = std::max(0.0, std::floor(mu - 3.0 * sd));
+    REQUIRE(mushroomCount >= static_cast<std::size_t>(lo));
+    REQUIRE(mushroomCount <= static_cast<std::size_t>(std::ceil(mu + 3.0 * sd)));
+}
+
+TEST_CASE("mushroom stream does not perturb the main stream", "[mushrooms][conformance]") {
+    // The mushroom stream is independent (seed ^ MUSHROOM_PRNG_SALT) so
+    // the main-stream first-blade values must still match the canonical.
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    REQUIRE(blades.size() > 0);
+    REQUIRE(blades[0].baseX == Approx(4.941073726820111).margin(1e-12));
+    REQUIRE(blades[0].height == Approx(24.469991818248864).margin(1e-12));
+}
+
+TEST_CASE("non-mushroom blades have zero mushroom fields", "[mushrooms]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    for (const Blade& b : blades) {
+        if (!b.isMushroom) {
+            REQUIRE(b.mushroomCapColorIdx   == 0);
+            REQUIRE(b.mushroomCapWidth      == 0.0);
+            REQUIRE(b.mushroomCapHeight     == 0.0);
+            REQUIRE(b.mushroomStemHeight    == 0.0);
+            REQUIRE(b.mushroomStemThickness == 0.0);
+        }
+    }
+}
+
+TEST_CASE("mushroom field ranges respect spec", "[mushrooms]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+    for (const Blade& b : blades) {
+        if (b.isMushroom) {
+            REQUIRE(b.mushroomCapColorIdx < MUSHROOM_PALETTE_SIZE);
+            REQUIRE(b.mushroomCapWidth      >= MUSHROOM_CAP_WIDTH_MIN);
+            REQUIRE(b.mushroomCapWidth      <  MUSHROOM_CAP_WIDTH_MAX);
+            REQUIRE(b.mushroomCapHeight     >= MUSHROOM_CAP_HEIGHT_MIN);
+            REQUIRE(b.mushroomCapHeight     <  MUSHROOM_CAP_HEIGHT_MAX);
+            REQUIRE(b.mushroomStemHeight    >= MUSHROOM_STEM_HEIGHT_MIN);
+            REQUIRE(b.mushroomStemHeight    <  MUSHROOM_STEM_HEIGHT_MAX);
+            REQUIRE(b.mushroomStemThickness >= MUSHROOM_STEM_THICKNESS_MIN);
+            REQUIRE(b.mushroomStemThickness <  MUSHROOM_STEM_THICKNESS_MAX);
+        }
+    }
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/ocean_tests.cpp

@@ -0,0 +1,114 @@
+// ocean_tests.cpp
+//
+// Ocean scene tests (architecture.md §17). Mirror of the Win2D OceanTests so
+// the coral blade variant, bubble emitter, and fish swimmers stay in lockstep
+// across impls.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+
+using namespace desktopgrass;
+
+namespace {
+
+constexpr double kMonitor1920 = 1920.0;
+
+Sim make_ocean_sim(uint64_t seed = CANONICAL_TEST_SEED,
+                   double width = kMonitor1920,
+                   double density = DEFAULT_DENSITY) {
+    Sim sim = sim_init(seed, width, density);
+    sim_set_scene(sim, Scene::Ocean);
+    return sim;
+}
+
+int count_kind(const Sim& sim, EntityKind kind) {
+    return static_cast<int>(std::count_if(sim.entities.begin(), sim.entities.end(),
+        [kind](const Entity& e) { return e.kind == kind; }));
+}
+
+} // namespace
+
+TEST_CASE("Ocean scene generates at least one coral and keeps values in range",
+          "[ocean][coral]") {
+    Sim sim = make_ocean_sim();
+
+    int coralCount = 0;
+    for (const Blade& b : sim.blades) {
+        if (!b.isCoral) continue;
+        ++coralCount;
+        REQUIRE_FALSE(b.isPine);
+        REQUIRE_FALSE(b.isCactus);
+        REQUIRE_FALSE(b.isMaple);
+        REQUIRE_FALSE(b.isFlower);
+        REQUIRE_FALSE(b.isMushroom);
+        REQUIRE(b.coralHeight >= CORAL_HEIGHT_MIN);
+        REQUIRE(b.coralHeight <= CORAL_HEIGHT_MAX);
+        REQUIRE(b.coralWidth  >= CORAL_WIDTH_MIN);
+        REQUIRE(b.coralWidth  <= CORAL_WIDTH_MAX);
+        REQUIRE(static_cast<int>(b.coralType)     >= 0);
+        REQUIRE(static_cast<int>(b.coralType)     <= CORAL_TYPE_COUNT  - 1);
+        REQUIRE(static_cast<int>(b.coralColorIdx) >= 0);
+        REQUIRE(static_cast<int>(b.coralColorIdx) <= CORAL_COLOR_COUNT - 1);
+    }
+    REQUIRE(coralCount > 0);
+}
+
+TEST_CASE("Ocean scene spawns initial fish at or above the target minimum",
+          "[ocean][fish]") {
+    Sim sim = make_ocean_sim();
+
+    const int fishCount = count_kind(sim, EntityKind::Fish);
+    REQUIRE(fishCount >= FISH_COUNT_MIN);
+    REQUIRE(fishCount <= FISH_COUNT_MAX);
+}
+
+TEST_CASE("Ocean fish count rounds half-to-even deterministically",
+          "[ocean][fish]") {
+    // scaled = 2.5 * width / 1920. Widths chosen so scaled lands exactly on a
+    // .5 tie; round-half-to-even must pick the even neighbor (NOT half-up),
+    // matching C# Math.Round and independent of the FPU rounding mode.
+    Sim tie25 = make_ocean_sim(CANONICAL_TEST_SEED, 1920.0); // scaled 2.5 -> 2
+    REQUIRE(count_kind(tie25, EntityKind::Fish) == 2);
+
+    Sim tie45 = make_ocean_sim(CANONICAL_TEST_SEED, 3456.0); // scaled 4.5 -> 4
+    REQUIRE(count_kind(tie45, EntityKind::Fish) == 4);
+}
+
+TEST_CASE("Ocean tick emits bubbles over time", "[ocean][bubble]") {
+    Sim sim = make_ocean_sim();
+
+    const double dt = 1.0 / 60.0;
+    for (int i = 0; i < 600; ++i) {
+        sim.globalTime += dt;
+        sim_tick_entities(sim, dt);
+    }
+
+    REQUIRE(count_kind(sim, EntityKind::Bubble) > 0);
+}
+
+TEST_CASE("Switching from Ocean to Grass wipes bubbles and fish",
+          "[ocean][scene]") {
+    Sim sim = make_ocean_sim();
+
+    const double dt = 1.0 / 60.0;
+    for (int i = 0; i < 120; ++i) {
+        sim.globalTime += dt;
+        sim_tick_entities(sim, dt);
+    }
+    REQUIRE(count_kind(sim, EntityKind::Fish) > 0);
+
+    sim_set_scene(sim, Scene::Grass);
+
+    REQUIRE(count_kind(sim, EntityKind::Bubble) == 0);
+    REQUIRE(count_kind(sim, EntityKind::Fish) == 0);
+    REQUIRE(std::none_of(sim.blades.begin(), sim.blades.end(),
+        [](const Blade& b) { return b.isCoral; }));
+}
+
+TEST_CASE("Ocean palette is pinned in scene palettes", "[ocean][palette]") {
+    for (int i = 0; i < PALETTE_SIZE; ++i) {
+        REQUIRE(SCENE_PALETTES[static_cast<int>(Scene::Ocean)][i] == OCEAN_PALETTE[i]);
+    }
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/pacing_tests.cpp

@@ -0,0 +1,80 @@
+// pacing_tests.cpp
+//
+// FramePacer behaviour tests.
+//
+// Goal: lock in the contract that on supported Windows (10 1803+) the pacer
+// honours sub-15.6 ms waits via the high-resolution waitable timer, not the
+// default system timer resolution. A regression that drops the high-res flag
+// would silently re-introduce the ~48 ms dt_p95 pacing bug; the timing-bound
+// assertion below catches that without needing benchmark numbers.
+//
+// The timing assertions are deliberately generous (we measure absolute upper
+// bounds, not exact wait times) so CI runners with momentary scheduling
+// hiccups don't flake. Even at the loosest bound the test still distinguishes
+// high-res (~sub-ms) from default-resolution (~15.6 ms minimum tick) behaviour.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Pacing.h"
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+using namespace desktopgrass;
+
+namespace {
+
+double qpc_now_sec() {
+    LARGE_INTEGER c{}, f{};
+    QueryPerformanceCounter(&c);
+    QueryPerformanceFrequency(&f);
+    return static_cast<double>(c.QuadPart) / static_cast<double>(f.QuadPart);
+}
+
+} // namespace
+
+TEST_CASE("FramePacer: creates a high-resolution waitable timer on supported Windows",
+          "[pacing]") {
+    FramePacer pacer;
+    // DesktopGrass requires Windows 10 1809+, which is well past the
+    // CREATE_WAITABLE_TIMER_HIGH_RESOLUTION minimum (Win 10 1803). Build/CI
+    // environments below that floor are not supported.
+    REQUIRE(pacer.IsHighResolution());
+}
+
+TEST_CASE("FramePacer: zero or negative wait returns essentially immediately",
+          "[pacing]") {
+    FramePacer pacer;
+    const double t0 = qpc_now_sec();
+    pacer.WaitUntilNextFrame(0.0);
+    pacer.WaitUntilNextFrame(-1.0);
+    const double dt = qpc_now_sec() - t0;
+    // Two no-op calls should complete in well under a millisecond, but allow
+    // 5 ms of slop for loaded CI machines.
+    REQUIRE(dt < 0.005);
+}
+
+TEST_CASE("FramePacer: honours sub-15.6 ms waits via the high-resolution timer",
+          "[pacing]") {
+    FramePacer pacer;
+    REQUIRE(pacer.IsHighResolution());
+
+    // Five 1 ms waits. With the high-resolution timer the cumulative time
+    // should sit well below 30 ms. Without it (legacy ~15.6 ms tick) each
+    // wait would round up to ~15.6 ms for a total of ~78 ms, so 30 ms is a
+    // wide safety margin that still catches regressions cleanly.
+    constexpr int    kIterations = 5;
+    constexpr double kWaitSec    = 0.001;
+
+    const double t0 = qpc_now_sec();
+    for (int i = 0; i < kIterations; ++i) {
+        pacer.WaitUntilNextFrame(kWaitSec);
+    }
+    const double total = qpc_now_sec() - t0;
+
+    // Lower bound: we asked for 5 ms total — actual wait must be at least
+    // a small fraction of that, otherwise we are not waiting at all.
+    REQUIRE(total >= 0.0005);
+    // Upper bound: must beat the default ~15.6 ms tick by a comfortable
+    // margin. 30 ms catches the regression (78 ms) without flaking on CI.
+    REQUIRE(total < 0.030);
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/persistence_tests.cpp

@@ -0,0 +1,285 @@
+#include "../third_party/catch2/catch.hpp"
+#include "Persistence.h"
+#include "Sim.h"
+
+#include <algorithm>
+#include <filesystem>
+#include <fstream>
+#include <sstream>
+#include <string>
+
+using namespace desktopgrass;
+
+namespace {
+
+std::filesystem::path test_state_path(const char* name) {
+    std::filesystem::path dir = std::filesystem::current_path()
+        / ".copilot-scratch"
+        / "native-persistence-tests"
+        / name;
+    std::error_code ec;
+    std::filesystem::remove_all(dir, ec);
+    std::filesystem::create_directories(dir);
+    return dir / "state.json";
+}
+
+void use_state_path(const std::filesystem::path& path) {
+    persistence::SetStateFilePathForTest(path.wstring());
+}
+
+void write_text(const std::filesystem::path& path, const std::string& text) {
+    std::filesystem::create_directories(path.parent_path());
+    std::ofstream file(path, std::ios::binary | std::ios::trunc);
+    file << text;
+}
+
+std::string read_text(const std::filesystem::path& path) {
+    std::ifstream file(path, std::ios::binary);
+    std::ostringstream buffer;
+    buffer << file.rdbuf();
+    return buffer.str();
+}
+
+persistence::AppState make_state_with_cuts() {
+    persistence::AppState state;
+    state.scene = Scene::Winter;
+    state.critter = CritterKind::Cat;
+    state.critterCountOverride = 4;
+    state.autoStart = true;
+
+    for (int i = 0; i < 3; ++i) {
+        persistence::MonitorState monitor;
+        monitor.width = 1920 + i * 320;
+        monitor.height = 1080 + i * 120;
+        monitor.left = i * 1920;
+        monitor.top = i == 2 ? -120 : 0;
+        const int cutCount = 2 + i;
+        for (int j = 0; j < cutCount; ++j) {
+            monitor.cuts.push_back(persistence::CutRecord{ i * 100 + j, -5.0 - i - j * 0.5 });
+        }
+        state.monitors.push_back(monitor);
+    }
+
+    return state;
+}
+
+void assert_state_equal(const persistence::AppState& expected, const persistence::AppState& actual) {
+    REQUIRE(actual.version == 2);
+    REQUIRE(actual.scene == expected.scene);
+    REQUIRE(actual.critter == expected.critter);
+    REQUIRE(actual.critterCountOverride == expected.critterCountOverride);
+    REQUIRE(actual.autoStart == expected.autoStart);
+    REQUIRE(actual.monitors.size() == expected.monitors.size());
+
+    for (std::size_t i = 0; i < expected.monitors.size(); ++i) {
+        const auto& e = expected.monitors[i];
+        const auto& a = actual.monitors[i];
+        REQUIRE(a.width == e.width);
+        REQUIRE(a.height == e.height);
+        REQUIRE(a.left == e.left);
+        REQUIRE(a.top == e.top);
+        REQUIRE(a.cuts.size() == e.cuts.size());
+        for (std::size_t j = 0; j < e.cuts.size(); ++j) {
+            REQUIRE(a.cuts[j].bladeIndex == e.cuts[j].bladeIndex);
+            REQUIRE(a.cuts[j].cutTime == Approx(e.cuts[j].cutTime).margin(1e-9));
+        }
+    }
+}
+
+Blade make_blade(double regrowDelay, double regrowDuration) {
+    Blade b{};
+    b.baseX = 100.0;
+    b.height = 20.0;
+    b.thickness = 1.0;
+    b.cutHeight = 1.0;
+    b.cutAnimStart = -1.0;
+    b.cutInitialHeight = 1.0;
+    b.regrowDelay = regrowDelay;
+    b.regrowDuration = regrowDuration;
+    b.regrowStart = -1.0;
+    return b;
+}
+
+} // namespace
+
+TEST_CASE("persistence round-trips empty state", "[persistence]") {
+    const auto path = test_state_path("round-trip-empty");
+    use_state_path(path);
+
+    persistence::AppState expected;
+    REQUIRE(persistence::SaveAppState(expected));
+
+    persistence::AppState actual;
+    REQUIRE(persistence::LoadAppState(actual));
+    assert_state_equal(expected, actual);
+}
+
+TEST_CASE("persistence round-trips state with cuts", "[persistence]") {
+    const auto path = test_state_path("round-trip-cuts");
+    use_state_path(path);
+
+    const persistence::AppState expected = make_state_with_cuts();
+    REQUIRE(persistence::SaveAppState(expected));
+
+    persistence::AppState actual;
+    REQUIRE(persistence::LoadAppState(actual));
+    assert_state_equal(expected, actual);
+}
+
+TEST_CASE("persistence round-trips every scene", "[persistence]") {
+    const Scene scenes[] = {
+        Scene::Grass, Scene::Desert, Scene::Winter, Scene::Autumn, Scene::Ocean
+    };
+    for (Scene scene : scenes) {
+        const auto path = test_state_path("round-trip-scene");
+        use_state_path(path);
+
+        persistence::AppState expected;
+        expected.scene = scene;
+        REQUIRE(persistence::SaveAppState(expected));
+
+        persistence::AppState actual;
+        REQUIRE(persistence::LoadAppState(actual));
+        REQUIRE(actual.scene == scene);
+    }
+}
+
+TEST_CASE("persistence version mismatch returns false", "[persistence]") {
+    const auto path = test_state_path("version-mismatch");
+    use_state_path(path);
+    write_text(path, "{ \"version\": 999, \"monitors\": {} }");
+
+    persistence::AppState actual;
+    REQUIRE_FALSE(persistence::LoadAppState(actual));
+}
+
+TEST_CASE("persistence missing file returns false", "[persistence]") {
+    const auto path = test_state_path("missing-file");
+    use_state_path(path);
+
+    persistence::AppState actual;
+    REQUIRE_FALSE(persistence::LoadAppState(actual));
+}
+
+TEST_CASE("persistence malformed json returns false", "[persistence]") {
+    const auto path = test_state_path("malformed-json");
+    use_state_path(path);
+    write_text(path, "not-json");
+
+    persistence::AppState actual;
+    REQUIRE_FALSE(persistence::LoadAppState(actual));
+}
+
+TEST_CASE("persistence atomic write leaves final file and removes tmp", "[persistence]") {
+    const auto path = test_state_path("atomic-write");
+    use_state_path(path);
+
+    persistence::AppState state;
+    REQUIRE(persistence::SaveAppState(state));
+
+    REQUIRE(std::filesystem::exists(path));
+    REQUIRE_FALSE(std::filesystem::exists(std::filesystem::path(path.wstring() + L".tmp")));
+}
+
+TEST_CASE("persistence monitor key format round-trips", "[persistence]") {
+    const auto path = test_state_path("monitor-key");
+    use_state_path(path);
+
+    persistence::AppState state;
+    persistence::MonitorState monitor;
+    monitor.width = 1920;
+    monitor.height = 1080;
+    monitor.left = 0;
+    monitor.top = 0;
+    state.monitors.push_back(monitor);
+
+    REQUIRE(persistence::SaveAppState(state));
+    REQUIRE(read_text(path).find("\"1920x1080@0,0\"") != std::string::npos);
+
+    persistence::AppState loaded;
+    REQUIRE(persistence::LoadAppState(loaded));
+    REQUIRE(loaded.monitors.size() == 1);
+    REQUIRE(persistence::MonitorKey(loaded.monitors[0]) == "1920x1080@0,0");
+}
+
+TEST_CASE("persistence cut timestamps shift for fresh sim load", "[persistence]") {
+    const auto path = test_state_path("time-shift");
+    use_state_path(path);
+
+    Sim running;
+    running.globalTime = 100.0;
+    running.blades.push_back(make_blade(30.0, 10.0));
+    running.blades[0].cutHeight = 0.0;
+    running.blades[0].regrowStart = 80.0 + CUT_DURATION_SEC + running.blades[0].regrowDelay;
+
+    auto cuts = sim_get_cuts(running);
+    REQUIRE(cuts.size() == 1);
+    REQUIRE(cuts[0].cutTime == Approx(-20.0).margin(1e-9));
+
+    persistence::AppState state;
+    persistence::MonitorState monitor;
+    monitor.width = 1920;
+    monitor.height = 1080;
+    monitor.left = 0;
+    monitor.top = 0;
+    monitor.cuts = cuts;
+    state.monitors.push_back(monitor);
+    REQUIRE(persistence::SaveAppState(state));
+
+    persistence::AppState loaded;
+    REQUIRE(persistence::LoadAppState(loaded));
+    REQUIRE(loaded.monitors[0].cuts[0].cutTime < 0.0);
+
+    Sim fresh;
+    fresh.globalTime = 0.0;
+    fresh.blades.push_back(make_blade(30.0, 10.0));
+    sim_apply_cuts(fresh, loaded.monitors[0].cuts);
+    REQUIRE(fresh.blades[0].cutHeight == Approx(0.0).margin(1e-9));
+    REQUIRE(fresh.blades[0].regrowStart == Approx(10.0 + CUT_DURATION_SEC).margin(1e-9));
+}
+
+TEST_CASE("persistence unmatched monitor cuts are skipped", "[persistence]") {
+    const auto path = test_state_path("unmatched-monitor");
+    use_state_path(path);
+
+    persistence::AppState state;
+    persistence::MonitorState unmatched;
+    unmatched.width = 9999;
+    unmatched.height = 9999;
+    unmatched.left = 99;
+    unmatched.top = 99;
+    unmatched.cuts.push_back(persistence::CutRecord{ 0, -20.0 });
+    state.monitors.push_back(unmatched);
+    REQUIRE(persistence::SaveAppState(state));
+
+    persistence::AppState loaded;
+    REQUIRE(persistence::LoadAppState(loaded));
+
+    Sim sim;
+    sim.blades.push_back(make_blade(30.0, 10.0));
+    const int width = 1920;
+    const int height = 1080;
+    const int left = 0;
+    const int top = 0;
+    const auto match = std::find_if(loaded.monitors.begin(), loaded.monitors.end(),
+        [&](const persistence::MonitorState& monitor) {
+            return monitor.width == width && monitor.height == height
+                && monitor.left == left && monitor.top == top;
+        });
+    if (match != loaded.monitors.end()) {
+        sim_apply_cuts(sim, match->cuts);
+    }
+
+    REQUIRE(sim_get_cuts(sim).empty());
+}
+
+TEST_CASE("persistence json is human readable", "[persistence]") {
+    const auto path = test_state_path("human-readable");
+    use_state_path(path);
+
+    REQUIRE(persistence::SaveAppState(make_state_with_cuts()));
+    const std::string text = read_text(path);
+    REQUIRE(text.find('\n') != std::string::npos);
+    REQUIRE(text.find("  \"version\"") != std::string::npos);
+    REQUIRE(text.find("    \"") != std::string::npos);
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/pine_tests.cpp

@@ -0,0 +1,255 @@
+// pine_tests.cpp - §15.1 Winter pine trees (slot-bound, mirrors §14 cacti).
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+#include "snapshot_data.h"
+
+#include <cmath>
+#include <cstddef>
+#include <vector>
+
+using namespace desktopgrass;
+
+namespace {
+
+constexpr double kMonitor1920 = 1920.0;
+
+struct ExpectedTree {
+    std::size_t slotIndex = 0;
+    uint8_t variant = 0;
+    double height = 0.0;
+    double width = 0.0;
+    int tierCount = 0;
+};
+
+ExpectedTree first_expected_tree(std::size_t bladeCount) {
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED ^ PINE_PRNG_SALT);
+
+    for (std::size_t i = 0; i < bladeCount; ++i) {
+        const double r = prng_uniform(p, 0.0, 1.0);
+        if (r >= PINE_PROBABILITY) continue;
+
+        ExpectedTree expected{};
+        expected.slotIndex = i;
+        const double variantDraw = prng_uniform(p, 0.0, 1.0);
+        expected.variant = variantDraw < BIRCH_VARIANT_PROBABILITY ? 1 : 0;
+        expected.height = prng_uniform(p, PINE_HEIGHT_MIN, PINE_HEIGHT_MAX);
+        if (expected.variant == 1) {
+            expected.width = prng_uniform(p, BIRCH_TRUNK_WIDTH_MIN, BIRCH_TRUNK_WIDTH_MAX);
+        } else {
+            expected.width = prng_uniform(p, PINE_WIDTH_MIN, PINE_WIDTH_MAX);
+        }
+        const double tierDraw = prng_uniform(p,
+            static_cast<double>(PINE_TIER_COUNT_MIN),
+            static_cast<double>(PINE_TIER_COUNT_MAX + 1));
+        int tiers = static_cast<int>(std::floor(tierDraw));
+        if (tiers < PINE_TIER_COUNT_MIN) tiers = PINE_TIER_COUNT_MIN;
+        if (tiers > PINE_TIER_COUNT_MAX) tiers = PINE_TIER_COUNT_MAX;
+        expected.tierCount = tiers;
+        return expected;
+    }
+
+    FAIL("canonical seed produced no tree slot");
+    return {};
+}
+
+} // anonymous
+
+TEST_CASE("Pine constants are pinned", "[pine][constants]") {
+    REQUIRE(PINE_PROBABILITY == Approx(0.0075));
+    REQUIRE(PINE_HEIGHT_MIN == Approx(45.0));
+    REQUIRE(PINE_HEIGHT_MAX == Approx(90.0));
+    REQUIRE(PINE_WIDTH_MIN  == Approx(28.0));
+    REQUIRE(PINE_WIDTH_MAX  == Approx(48.0));
+    REQUIRE(PINE_TIER_COUNT_MIN == 2);
+    REQUIRE(PINE_TIER_COUNT_MAX == 4);
+    REQUIRE(PINE_TIP_TAPER == Approx(0.25));
+    REQUIRE(PINE_TIER_OVERLAP == Approx(0.15));
+    REQUIRE(PINE_SNOW_CAP_FRACTION == Approx(0.30));
+    REQUIRE(PINE_COLOR == 0xFF1B5E20u);
+    REQUIRE(PINE_PRNG_SALT == 0x50494E4550494E45ull);
+}
+
+TEST_CASE("Birch constants are pinned", "[pine][birch][constants]") {
+    REQUIRE(BIRCH_VARIANT_PROBABILITY == Approx(0.30));
+    REQUIRE(BIRCH_TRUNK_WIDTH_MIN == Approx(4.0));
+    REQUIRE(BIRCH_TRUNK_WIDTH_MAX == Approx(7.0));
+    REQUIRE(BIRCH_BARK_MARK_COUNT == 5);
+    REQUIRE(BIRCH_BARK_MARK_LENGTH_FRAC == Approx(0.50));
+    REQUIRE(BIRCH_BRANCH_COUNT == 6);
+    REQUIRE(BIRCH_SNOW_CAP_FRACTION == Approx(0.18));
+    REQUIRE(BIRCH_BARK_COLOR == 0xFFEFEFE6u);
+    REQUIRE(BIRCH_MARK_COLOR == 0xFF2A2A28u);
+}
+
+TEST_CASE("sim_set_scene Winter promotes some slots to trees", "[pine][scene]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Winter);
+
+    REQUIRE(sim.currentScene == Scene::Winter);
+    std::size_t treeCount = 0;
+    for (const Blade& b : sim.blades) {
+        if (b.isPine) {
+            ++treeCount;
+            REQUIRE(b.pineTierCount >= PINE_TIER_COUNT_MIN);
+            REQUIRE(b.pineTierCount <= PINE_TIER_COUNT_MAX);
+            REQUIRE(b.pineHeight >= PINE_HEIGHT_MIN);
+            REQUIRE(b.pineHeight <= PINE_HEIGHT_MAX);
+            const double widthMin = (b.treeVariant == 1) ? BIRCH_TRUNK_WIDTH_MIN : PINE_WIDTH_MIN;
+            const double widthMax = (b.treeVariant == 1) ? BIRCH_TRUNK_WIDTH_MAX : PINE_WIDTH_MAX;
+            REQUIRE(b.pineWidth >= widthMin);
+            REQUIRE(b.pineWidth <= widthMax);
+        }
+    }
+    REQUIRE(treeCount >= 1);
+    REQUIRE(treeCount <= 25);
+}
+
+TEST_CASE("First tree matches the spec-derived PRNG snapshot", "[pine][snapshot]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    const ExpectedTree expected = first_expected_tree(sim.blades.size());
+
+    sim_set_scene(sim, Scene::Winter);
+
+    REQUIRE(expected.slotIndex < sim.blades.size());
+    const Blade& b = sim.blades[expected.slotIndex];
+    REQUIRE(b.isPine);
+    REQUIRE(b.treeVariant == expected.variant);
+    REQUIRE(b.pineHeight == Approx(expected.height).margin(1e-12));
+    REQUIRE(b.pineWidth  == Approx(expected.width).margin(1e-12));
+    REQUIRE(b.pineTierCount == expected.tierCount);
+}
+
+TEST_CASE("Grass scene restores tree slots to vanilla variants", "[pine][restore]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    const ExpectedTree expected = first_expected_tree(sim.blades.size());
+    REQUIRE(expected.slotIndex < sim.blades.size());
+
+    Blade& target = sim.blades[expected.slotIndex];
+    target.isFlower = true;
+    target.isMushroom = true;
+    target.originalIsFlower = true;
+    target.originalIsMushroom = true;
+
+    sim_set_scene(sim, Scene::Winter);
+    REQUIRE(sim.blades[expected.slotIndex].isPine);
+    REQUIRE_FALSE(sim.blades[expected.slotIndex].isFlower);
+    REQUIRE_FALSE(sim.blades[expected.slotIndex].isMushroom);
+
+    sim_set_scene(sim, Scene::Grass);
+    REQUIRE_FALSE(sim.blades[expected.slotIndex].isPine);
+    REQUIRE(sim.blades[expected.slotIndex].treeVariant == 0);
+    REQUIRE(sim.blades[expected.slotIndex].isFlower);
+    REQUIRE(sim.blades[expected.slotIndex].isMushroom);
+}
+
+TEST_CASE("Winter produces both pine and birch variants over canonical seed", "[pine][birch]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Winter);
+
+    std::size_t pineCount = 0;
+    std::size_t birchCount = 0;
+    for (const Blade& b : sim.blades) {
+        if (!b.isPine) continue;
+        if (b.treeVariant == 0) {
+            ++pineCount;
+            REQUIRE(b.pineWidth >= PINE_WIDTH_MIN);
+            REQUIRE(b.pineWidth <= PINE_WIDTH_MAX);
+        } else {
+            REQUIRE(b.treeVariant == 1);
+            ++birchCount;
+            REQUIRE(b.pineWidth >= BIRCH_TRUNK_WIDTH_MIN);
+            REQUIRE(b.pineWidth <= BIRCH_TRUNK_WIDTH_MAX);
+        }
+    }
+    REQUIRE(pineCount >= 1);
+    REQUIRE(birchCount >= 1);
+}
+
+TEST_CASE("Winter scene suppresses mushrooms on every slot", "[pine][winter][mushroom]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    // Pre-mark a handful of slots as mushrooms; Winter must clear them all.
+    for (std::size_t i = 0; i < sim.blades.size(); i += 17) {
+        sim.blades[i].isMushroom = true;
+        sim.blades[i].originalIsMushroom = true;
+    }
+
+    sim_set_scene(sim, Scene::Winter);
+
+    for (const Blade& b : sim.blades) REQUIRE_FALSE(b.isMushroom);
+
+    // Switching back to Grass must restore the original mushroom flags.
+    sim_set_scene(sim, Scene::Grass);
+    REQUIRE(sim.blades[0].isMushroom == sim.blades[0].originalIsMushroom);
+}
+
+TEST_CASE("Winter grass height scale is pinned", "[pine][winter][scale]") {
+    REQUIRE(WINTER_GRASS_HEIGHT_SCALE == Approx(0.5));
+}
+
+TEST_CASE("Tree depth constants are pinned", "[pine][depth][constants]") {
+    REQUIRE(TREE_BACKGROUND_PROBABILITY == Approx(0.45));
+    REQUIRE(TREE_BG_SCALE == Approx(0.62));
+    REQUIRE(TREE_BG_OPACITY == Approx(0.78f));
+}
+
+TEST_CASE("Winter mixes foreground and background trees", "[pine][depth]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Winter);
+
+    std::size_t fg = 0;
+    std::size_t bg = 0;
+    for (const Blade& b : sim.blades) {
+        if (!b.isPine) continue;
+        if (b.treeBackground) ++bg; else ++fg;
+    }
+    REQUIRE(fg >= 1);
+    REQUIRE(bg >= 1);
+}
+
+TEST_CASE("Tree depth assignment is deterministic across re-entry", "[pine][depth]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Winter);
+
+    std::vector<bool> firstPass;
+    for (const Blade& b : sim.blades) {
+        if (b.isPine) firstPass.push_back(b.treeBackground);
+    }
+
+    // Leaving and re-entering Winter must reproduce the same depth layout.
+    sim_set_scene(sim, Scene::Grass);
+    sim_set_scene(sim, Scene::Winter);
+
+    std::size_t idx = 0;
+    for (const Blade& b : sim.blades) {
+        if (!b.isPine) continue;
+        REQUIRE(idx < firstPass.size());
+        REQUIRE(b.treeBackground == firstPass[idx]);
+        ++idx;
+    }
+    REQUIRE(idx == firstPass.size());
+}
+
+TEST_CASE("Non-winter scenes clear the tree background flag", "[pine][depth][restore]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Winter);
+    sim_set_scene(sim, Scene::Grass);
+    for (const Blade& b : sim.blades) REQUIRE_FALSE(b.treeBackground);
+}
+
+TEST_CASE("Winter scene leaves the canonical first blade geometry bit-identical", "[pine][snapshot]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, kMonitor1920, 1.0);
+    REQUIRE(sim.blades.size() == desktopgrass::test::CANONICAL_BLADE_COUNT);
+
+    sim_set_scene(sim, Scene::Winter);
+
+    const Blade& first = sim.blades[0];
+    const auto& expected = desktopgrass::test::CANONICAL_FIRST_10[0];
+    REQUIRE(first.baseX == Approx(expected.baseX).margin(1e-12));
+    REQUIRE(first.height == Approx(expected.height).margin(1e-12));
+    REQUIRE(first.thickness == Approx(expected.thickness).margin(1e-12));
+    REQUIRE(first.hue == expected.hue);
+    REQUIRE(first.swayPhaseOffset == Approx(expected.sway).margin(1e-12));
+    REQUIRE(first.stiffness == Approx(expected.stiffness).margin(1e-12));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/prng_tests.cpp

@@ -0,0 +1,78 @@
+// prng_tests.cpp
+//
+// Conformance + snapshot tests for the PRNG (architecture.md §3).
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+#include "snapshot_data.h"
+
+using namespace desktopgrass;
+using namespace desktopgrass::test;
+
+TEST_CASE("PRNG matches the canonical 16-output snapshot", "[prng][snapshot]") {
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED);
+
+    for (std::size_t i = 0; i < 16; ++i) {
+        uint64_t v = prng_next_u64(p);
+        INFO("index = " << i);
+        REQUIRE(v == CANONICAL_PRNG_SNAPSHOT[i]);
+    }
+}
+
+TEST_CASE("PRNG is deterministic for a given seed", "[prng]") {
+    Prng a, b;
+    prng_init(a, CANONICAL_TEST_SEED);
+    prng_init(b, CANONICAL_TEST_SEED);
+    for (int i = 0; i < 1000; ++i) {
+        REQUIRE(prng_next_u64(a) == prng_next_u64(b));
+    }
+}
+
+TEST_CASE("PRNG decorrelates seed=0 via splitmix64", "[prng]") {
+    // seed == 0 must not produce a stuck-at-zero PRNG.
+    Prng p;
+    prng_init(p, 0);
+    REQUIRE(p.state != 0);
+    uint64_t a = prng_next_u64(p);
+    uint64_t b = prng_next_u64(p);
+    REQUIRE(a != 0);
+    REQUIRE(b != 0);
+    REQUIRE(a != b);
+}
+
+TEST_CASE("prng_next_unit is in [0, 1)", "[prng]") {
+    Prng p;
+    prng_init(p, CANONICAL_TEST_SEED);
+    for (int i = 0; i < 10000; ++i) {
+        double u = prng_next_unit(p);
+        REQUIRE(u >= 0.0);
+        REQUIRE(u <  1.0);
+    }
+}
+
+TEST_CASE("prng_uniform stays within [lo, hi)", "[prng]") {
+    Prng p;
+    prng_init(p, 12345);
+    for (int i = 0; i < 10000; ++i) {
+        double v = prng_uniform(p, 8.0, 40.0);
+        REQUIRE(v >= 8.0);
+        REQUIRE(v <  40.0);
+    }
+}
+
+TEST_CASE("prng_index is in [0, n)", "[prng]") {
+    Prng p;
+    prng_init(p, 42);
+    bool sawZero = false;
+    bool sawFive = false;
+    for (int i = 0; i < 10000; ++i) {
+        uint32_t v = prng_index(p, PALETTE_SIZE);
+        REQUIRE(v < PALETTE_SIZE);
+        if (v == 0) sawZero = true;
+        if (v == 5) sawFive = true;
+    }
+    // Distribution sanity. Not strict — just confirms we cover both extremes.
+    REQUIRE(sawZero);
+    REQUIRE(sawFive);
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/prop_spacing_tests.cpp

@@ -0,0 +1,136 @@
+#include "catch.hpp"
+
+#include "Sim.h"
+#include "Constants.h"
+
+#include <vector>
+
+using namespace desktopgrass;
+
+namespace {
+
+constexpr double kMonitor1920 = 1920.0;
+constexpr double kDensity     = 1.0;
+constexpr uint64_t kSeed      = 0xDE5C70F6A55ED511ull;
+
+struct Prop {
+    double leftEdge;
+    double rightEdge;
+};
+
+double cactus_half_width(const Blade& b) {
+    return (b.cactusType != 0) ? b.cactusWidth * 1.55 : b.cactusWidth * 0.5;
+}
+
+double pine_half_width(const Blade& b) {
+    double hw = (b.treeVariant == 1) ? b.pineWidth * 4.0 : b.pineWidth * 0.5;
+    if (b.treeBackground) hw *= TREE_BG_SCALE;
+    return hw;
+}
+
+// Walk the prop list left-to-right and verify that every adjacent pair has
+// at least PROP_MIN_GAP_DIP between the right edge of one and the left edge
+// of the next. The generators emit props in baseX order so a single linear
+// pass is sufficient.
+void require_spacing(const std::vector<Prop>& props, double minGap, const char* label) {
+    INFO(label << ": " << props.size() << " props placed");
+    REQUIRE(props.size() >= 1);
+    for (std::size_t i = 1; i < props.size(); ++i) {
+        const double gap = props[i].leftEdge - props[i - 1].rightEdge;
+        INFO("pair " << (i - 1) << "→" << i
+             << " right=" << props[i - 1].rightEdge
+             << " left=" << props[i].leftEdge
+             << " gap=" << gap);
+        REQUIRE(gap >= minGap);
+    }
+}
+
+}  // namespace
+
+TEST_CASE("Desert cacti keep at least PROP_MIN_GAP_DIP between neighbours",
+          "[spacing][desert][cactus]") {
+    Sim sim = sim_init(kSeed, kMonitor1920, kDensity);
+    sim_set_scene(sim, Scene::Desert);
+
+    std::vector<Prop> cacti;
+    for (const Blade& b : sim.blades) {
+        if (!b.isCactus) continue;
+        const double hw = cactus_half_width(b);
+        cacti.push_back({b.baseX - hw, b.baseX + hw});
+    }
+    require_spacing(cacti, PROP_MIN_GAP_DIP, "cacti");
+}
+
+TEST_CASE("Winter pines keep at least PROP_MIN_GAP_DIP between same-layer neighbours",
+          "[spacing][winter][pine]") {
+    Sim sim = sim_init(kSeed, kMonitor1920, kDensity);
+    sim_set_scene(sim, Scene::Winter);
+
+    std::vector<Prop> fgPines;
+    std::vector<Prop> bgPines;
+    for (const Blade& b : sim.blades) {
+        if (!b.isPine) continue;
+        const double hw = pine_half_width(b);
+        Prop p{b.baseX - hw, b.baseX + hw};
+        if (b.treeBackground) bgPines.push_back(p);
+        else                  fgPines.push_back(p);
+    }
+    require_spacing(fgPines, PROP_MIN_GAP_DIP, "foreground pines");
+    require_spacing(bgPines, PROP_MIN_GAP_DIP, "background pines");
+}
+
+TEST_CASE("Autumn maples keep at least PROP_MIN_GAP_DIP between neighbours",
+          "[spacing][autumn][maple]") {
+    Sim sim = sim_init(kSeed, kMonitor1920, kDensity);
+    sim_set_scene(sim, Scene::Autumn);
+
+    std::vector<Prop> maples;
+    for (const Blade& b : sim.blades) {
+        if (!b.isMaple) continue;
+        const double hw = b.mapleCanopyRadius;
+        maples.push_back({b.baseX - hw, b.baseX + hw});
+    }
+    require_spacing(maples, PROP_MIN_GAP_DIP, "maples");
+}
+
+TEST_CASE("Ocean coral keep at least PROP_MIN_GAP_DIP between neighbours",
+          "[spacing][ocean][coral]") {
+    Sim sim = sim_init(kSeed, kMonitor1920, kDensity);
+    sim_set_scene(sim, Scene::Ocean);
+
+    std::vector<Prop> coral;
+    for (const Blade& b : sim.blades) {
+        if (!b.isCoral) continue;
+        const double hw = b.coralWidth * 0.5;
+        coral.push_back({b.baseX - hw, b.baseX + hw});
+    }
+    require_spacing(coral, PROP_MIN_GAP_DIP, "coral");
+}
+
+TEST_CASE("Prop spacing rule reduces but doesn't decimate the population",
+          "[spacing][population]") {
+    // Sanity check that gap rejection isn't aggressive enough to break the
+    // existing "near-spec probability" tests — each scene should still place
+    // at least a handful of props on a 1920-DIP window with canonical seed.
+    Sim sim = sim_init(kSeed, kMonitor1920, kDensity);
+
+    sim_set_scene(sim, Scene::Desert);
+    int cactusCount = 0;
+    for (const Blade& b : sim.blades) if (b.isCactus) ++cactusCount;
+    REQUIRE(cactusCount >= 1);
+
+    sim_set_scene(sim, Scene::Winter);
+    int pineCount = 0;
+    for (const Blade& b : sim.blades) if (b.isPine) ++pineCount;
+    REQUIRE(pineCount >= 3);
+
+    sim_set_scene(sim, Scene::Autumn);
+    int mapleCount = 0;
+    for (const Blade& b : sim.blades) if (b.isMaple) ++mapleCount;
+    REQUIRE(mapleCount >= 1);
+
+    sim_set_scene(sim, Scene::Ocean);
+    int coralCount = 0;
+    for (const Blade& b : sim.blades) if (b.isCoral) ++coralCount;
+    REQUIRE(coralCount >= 5);
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/regrowth_tests.cpp

@@ -0,0 +1,196 @@
+// regrowth_tests.cpp
+//
+// Regrowth lifecycle tests (architecture.md §9 "Regrowth").
+//
+// Lifecycle: alive (cutHeight=1) -> cut anim (0.2s) -> stump (cutHeight=0,
+// regrowStart scheduled) -> wait regrowDelay -> regrow (linear over
+// regrowDuration) -> alive again.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <cmath>
+
+using namespace desktopgrass;
+
+namespace {
+
+// A test blade that opts in to regrowth — sets delay and duration to small,
+// known values so we can deterministically tick through the lifecycle.
+Blade make_regrowing_blade(double baseX, double regrowDelay, double regrowDuration) {
+    Blade b{};
+    b.baseX            = baseX;
+    b.height           = 20.0;
+    b.thickness        = 1.5;
+    b.swayPhaseOffset  = 0.0;
+    b.stiffness        = 1.0;
+    b.cutHeight        = 1.0;
+    b.cutInitialHeight = 1.0;
+    b.cutAnimStart     = -1.0;
+    b.regrowDelay      = regrowDelay;
+    b.regrowDuration   = regrowDuration;
+    b.regrowStart      = -1.0;
+    return b;
+}
+
+Sim make_sim_with(Blade b) {
+    Sim sim;
+    sim.windowHeight = STRIP_HEIGHT + HEADROOM;
+    sim.blades.push_back(b);
+    return sim;
+}
+
+InputEvent click(double x, double y, double t) {
+    return InputEvent{ EventType::Click, x, y, t };
+}
+
+} // anonymous
+
+TEST_CASE("cut completion schedules regrowth", "[regrowth]") {
+    Sim sim = make_sim_with(make_regrowing_blade(100.0, /*delay=*/1.0, /*dur=*/0.5));
+    const double y = sim.windowHeight - 40.0;
+
+    InputEvent ev = click(100.0, y, 0.0);
+    sim_tick(sim, 0.0, &ev, 1);
+
+    // Run the cut animation to completion (200 ms).
+    for (int i = 0; i < 4; ++i) sim_tick(sim, 0.05, nullptr, 0);
+
+    REQUIRE(sim.blades[0].cutHeight    == Approx(0.0));
+    REQUIRE(sim.blades[0].cutAnimStart  < 0.0);
+    // regrowStart is scheduled at globalTime + regrowDelay = 0.2 + 1.0 = 1.2.
+    REQUIRE(sim.blades[0].regrowStart  == Approx(1.2).margin(1e-9));
+}
+
+TEST_CASE("regrowth is linear over regrowDuration", "[regrowth]") {
+    Sim sim = make_sim_with(make_regrowing_blade(100.0, /*delay=*/0.5, /*dur=*/0.4));
+    const double y = sim.windowHeight - 40.0;
+
+    InputEvent ev = click(100.0, y, 0.0);
+    sim_tick(sim, 0.0, &ev, 1);
+
+    // Cut animation: 4 x 50 ms -> globalTime=0.20, cutHeight=0, regrowStart=0.70.
+    for (int i = 0; i < 4; ++i) sim_tick(sim, 0.05, nullptr, 0);
+    REQUIRE(sim.blades[0].regrowStart == Approx(0.70).margin(1e-9));
+
+    // Tick through the regrow delay (0.5s = 10 frames). Blade stays cut.
+    for (int i = 0; i < 10; ++i) {
+        sim_tick(sim, 0.05, nullptr, 0);
+        REQUIRE(sim.blades[0].cutHeight == Approx(0.0).margin(1e-9));
+    }
+    // globalTime = 0.70 now (start of regrowth).
+
+    // Quarter of the way through regrowth (dur=0.4 -> 0.10 elapsed): cutHeight = 0.25.
+    sim_tick(sim, 0.10, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight == Approx(0.25).margin(1e-9));
+
+    // Half way: cutHeight = 0.5.
+    sim_tick(sim, 0.10, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight == Approx(0.5).margin(1e-9));
+
+    // Three quarters: cutHeight = 0.75.
+    sim_tick(sim, 0.10, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight == Approx(0.75).margin(1e-9));
+
+    // Full: cutHeight = 1.0, regrowStart idle.
+    sim_tick(sim, 0.10, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight  == Approx(1.0).margin(1e-9));
+    REQUIRE(sim.blades[0].regrowStart  < 0.0);
+
+    // After regrowth, further ticks don't change cutHeight.
+    sim_tick(sim, 1.0, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight  == Approx(1.0).margin(1e-9));
+}
+
+TEST_CASE("re-click during regrowth restarts the cut from current height", "[regrowth]") {
+    Sim sim = make_sim_with(make_regrowing_blade(100.0, /*delay=*/0.1, /*dur=*/0.4));
+    const double y = sim.windowHeight - 40.0;
+
+    InputEvent ev1 = click(100.0, y, 0.0);
+    sim_tick(sim, 0.0, &ev1, 1);
+
+    // Drive the cut to completion + delay + halfway through regrowth.
+    // 4 ticks of 50 ms = 200 ms (cut done), globalTime=0.20, regrowStart=0.30.
+    for (int i = 0; i < 4; ++i) sim_tick(sim, 0.05, nullptr, 0);
+    // 2 ticks of 50 ms = 100 ms further -> globalTime=0.30 (regrowth starts).
+    for (int i = 0; i < 2; ++i) sim_tick(sim, 0.05, nullptr, 0);
+    // 4 ticks of 50 ms = 200 ms into the 0.4s regrowth -> cutHeight should be 0.5.
+    for (int i = 0; i < 4; ++i) sim_tick(sim, 0.05, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight == Approx(0.5).margin(1e-9));
+    REQUIRE(sim.blades[0].regrowStart > 0.0);
+
+    // Click again mid-regrowth.
+    InputEvent ev2 = click(100.0, y, 0.5);
+    sim_tick(sim, 0.0, &ev2, 1);
+
+    // Cut should restart: cutAnimStart valid, cutInitialHeight = 0.5,
+    // regrowStart cleared.
+    REQUIRE(sim.blades[0].cutAnimStart    >= 0.0);
+    REQUIRE(sim.blades[0].cutInitialHeight == Approx(0.5).margin(1e-9));
+    REQUIRE(sim.blades[0].regrowStart      < 0.0);
+
+    // Animate cut for 200 ms -> cutHeight returns to 0 and regrowth re-schedules.
+    for (int i = 0; i < 4; ++i) sim_tick(sim, 0.05, nullptr, 0);
+    REQUIRE(sim.blades[0].cutHeight    == Approx(0.0).margin(1e-9));
+    REQUIRE(sim.blades[0].cutAnimStart  < 0.0);
+    REQUIRE(sim.blades[0].regrowStart   > 0.0);
+}
+
+TEST_CASE("click on stump (cut, waiting to regrow) is a no-op", "[regrowth]") {
+    // cutHeight=0 and regrowStart scheduled but not yet started.
+    Sim sim = make_sim_with(make_regrowing_blade(100.0, /*delay=*/10.0, /*dur=*/1.0));
+    sim.blades[0].cutHeight    = 0.0;
+    sim.blades[0].cutAnimStart = -1.0;
+    sim.blades[0].regrowStart  = 5.0;  // scheduled
+
+    const double y = sim.windowHeight - 40.0;
+    InputEvent ev = click(100.0, y, 0.0);
+    sim_tick(sim, 0.0, &ev, 1);
+
+    REQUIRE(sim.blades[0].cutHeight    == Approx(0.0));
+    REQUIRE(sim.blades[0].cutAnimStart  < 0.0);
+    REQUIRE(sim.blades[0].regrowStart  == Approx(5.0));
+}
+
+TEST_CASE("regrowth jitter is deterministic for a given seed", "[regrowth][snapshot]") {
+    std::vector<Blade> a;
+    std::vector<Blade> b;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, a);
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, b);
+
+    REQUIRE(a.size() == b.size());
+    for (std::size_t i = 0; i < a.size(); ++i) {
+        REQUIRE(a[i].regrowDelay    == Approx(b[i].regrowDelay   ).margin(1e-12));
+        REQUIRE(a[i].regrowDuration == Approx(b[i].regrowDuration).margin(1e-12));
+    }
+}
+
+TEST_CASE("regrowth jitter falls within configured min/max", "[regrowth][snapshot]") {
+    std::vector<Blade> blades;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, blades);
+
+    REQUIRE(blades.size() > 50);
+    for (const Blade& b : blades) {
+        REQUIRE(b.regrowDelay    >= REGROW_DELAY_MIN);
+        REQUIRE(b.regrowDelay    <  REGROW_DELAY_MAX);
+        REQUIRE(b.regrowDuration >= REGROW_DURATION_MIN);
+        REQUIRE(b.regrowDuration <  REGROW_DURATION_MAX);
+        REQUIRE(b.regrowStart    == Approx(-1.0));
+    }
+}
+
+TEST_CASE("regrowth jitter does not perturb static-field generation", "[regrowth][snapshot]") {
+    // Whole point of the salted second-stream design: snapshot tests for
+    // baseX/height/etc are unaffected by adding regrowth. Cross-check by
+    // generating with and without regrowth jitter via two seeds that share
+    // the main stream but differ in regrow stream (i.e. same seed produces
+    // identical static fields).
+    std::vector<Blade> a;
+    generate_blades(CANONICAL_TEST_SEED, 1920.0, 1.0, a);
+
+    // Spec gates the static-field count + per-blade values; this is here
+    // as a tripwire if anyone slips an extra prng_next_* call into the
+    // main stream during generation.
+    REQUIRE(a.size() > 0);
+    REQUIRE(a[0].baseX    == Approx(a[0].baseX));   // tautology — placeholder
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/scene_tests.cpp

@@ -0,0 +1,105 @@
+// scene_tests.cpp
+//
+// Scene infrastructure tests (architecture.md §13).
+//
+// Coverage:
+//   * Scene enum discriminants match the spec ({Grass=0, Desert=1, Winter=2, Autumn=3}).
+//   * sim_init defaults currentScene to SCENE_DEFAULT (= Grass).
+//   * sim_set_scene does not perturb blade positions/dimensions/hues or
+//     any non-scene PRNG stream.
+//   * Per-scene palette tables are 6 entries each with full-alpha ARGB.
+//   * SCENE_PALETTES[Grass] is bit-identical to the original §4 PALETTE.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+#include "snapshot_data.h"
+
+#include <cstdint>
+
+using namespace desktopgrass;
+
+TEST_CASE("Scene enum has spec-locked discriminants", "[scene][enum]") {
+    REQUIRE(static_cast<int>(Scene::Grass)  == 0);
+    REQUIRE(static_cast<int>(Scene::Desert) == 1);
+    REQUIRE(static_cast<int>(Scene::Winter) == 2);
+    REQUIRE(static_cast<int>(Scene::Autumn) == 3);
+    REQUIRE(static_cast<int>(Scene::Ocean)  == 4);
+    REQUIRE(SCENE_COUNT == 5);
+    REQUIRE(static_cast<int>(SCENE_DEFAULT) == 0);
+}
+
+TEST_CASE("sim_init defaults currentScene to Grass", "[scene][init]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    REQUIRE(sim.currentScene == Scene::Grass);
+}
+
+TEST_CASE("sim_set_scene does not perturb blade geometry or hues", "[scene][independence]") {
+    Sim a = sim_init(CANONICAL_TEST_SEED, 1920.0, 1.0);
+    Sim b = sim_init(CANONICAL_TEST_SEED, 1920.0, 1.0);
+
+    // Same seed → same blades initially.
+    REQUIRE(a.blades.size() == b.blades.size());
+    REQUIRE(a.blades.size() == desktopgrass::test::CANONICAL_BLADE_COUNT);
+
+    sim_set_scene(b, Scene::Desert);
+
+    REQUIRE(b.currentScene == Scene::Desert);
+    REQUIRE(a.currentScene == Scene::Grass);
+    REQUIRE(a.blades.size() == b.blades.size());
+    for (size_t i = 0; i < a.blades.size(); ++i) {
+        REQUIRE(a.blades[i].baseX     == Approx(b.blades[i].baseX));
+        REQUIRE(a.blades[i].height    == Approx(b.blades[i].height));
+        REQUIRE(a.blades[i].thickness == Approx(b.blades[i].thickness));
+        REQUIRE(a.blades[i].hue       == b.blades[i].hue);
+    }
+    // Desert cacti may mutate variant tags, but geometry and ambient PRNG stay untouched.
+    REQUIRE(a.ambientPrng.state == b.ambientPrng.state);
+    REQUIRE(a.nextAmbientGustTime == Approx(b.nextAmbientGustTime));
+}
+
+TEST_CASE("sim_set_scene round-trips through all values", "[scene][api]") {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim_set_scene(sim, Scene::Desert);  REQUIRE(sim.currentScene == Scene::Desert);
+    sim_set_scene(sim, Scene::Winter);  REQUIRE(sim.currentScene == Scene::Winter);
+    sim_set_scene(sim, Scene::Autumn);  REQUIRE(sim.currentScene == Scene::Autumn);
+    sim_set_scene(sim, Scene::Ocean);   REQUIRE(sim.currentScene == Scene::Ocean);
+    sim_set_scene(sim, Scene::Grass);   REQUIRE(sim.currentScene == Scene::Grass);
+}
+
+TEST_CASE("Per-scene palette tables are 6 ARGB entries with full alpha", "[scene][palette]") {
+    for (int s = 0; s < SCENE_COUNT; ++s) {
+        for (int i = 0; i < PALETTE_SIZE; ++i) {
+            const uint32_t argb = SCENE_PALETTES[s][i];
+            const uint8_t alpha = static_cast<uint8_t>((argb >> 24) & 0xFFu);
+            REQUIRE(alpha == 0xFFu);
+        }
+    }
+}
+
+TEST_CASE("Grass scene palette is bit-identical to the original §4 PALETTE", "[scene][palette]") {
+    for (int i = 0; i < PALETTE_SIZE; ++i) {
+        REQUIRE(SCENE_PALETTES[static_cast<int>(Scene::Grass)][i] == PALETTE[i]);
+    }
+}
+
+TEST_CASE("Desert palette values match spec §13", "[scene][palette]") {
+    constexpr uint32_t expected[PALETTE_SIZE] = {
+        0xFFC9A26Bu, 0xFFB48A56u, 0xFFD9B57Au,
+        0xFF8F6E3Fu, 0xFFE6C896u, 0xFFA67843u,
+    };
+    for (int i = 0; i < PALETTE_SIZE; ++i) {
+        REQUIRE(SCENE_PALETTES[static_cast<int>(Scene::Desert)][i] == expected[i]);
+        REQUIRE(DESERT_PALETTE[i] == expected[i]);
+    }
+}
+
+TEST_CASE("Winter palette values match spec §13", "[scene][palette]") {
+    constexpr uint32_t expected[PALETTE_SIZE] = {
+        0xFFE8EEF5u, 0xFFB7C4D2u, 0xFFCBD8E5u,
+        0xFFD7E2EEu, 0xFFA8B7C6u, 0xFFEEF3F8u,
+    };
+    for (int i = 0; i < PALETTE_SIZE; ++i) {
+        REQUIRE(SCENE_PALETTES[static_cast<int>(Scene::Winter)][i] == expected[i]);
+        REQUIRE(WINTER_PALETTE[i] == expected[i]);
+    }
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/sheep_greeting_tests.cpp

@@ -0,0 +1,228 @@
+// sheep_greeting_tests.cpp
+//
+// §16 sheep proximity-greeting tests. Mirrors Win2D SheepGreetingTests.cs.
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cstddef>
+#include <vector>
+
+using namespace desktopgrass;
+
+namespace {
+
+constexpr double Monitor1920 = 1920.0;
+constexpr double EligibleAge = 2.0;
+constexpr double LongTimer = 10.0;
+
+Sim build_sheep_sim() {
+    Sim sim = sim_init(CANONICAL_TEST_SEED, Monitor1920, DEFAULT_DENSITY);
+    sim_set_critter(sim, CritterKind::Sheep);
+    return sim;
+}
+
+std::vector<std::size_t> sheep_indices(const Sim& sim) {
+    std::vector<std::size_t> indices;
+    for (std::size_t i = 0; i < sim.entities.size(); ++i) {
+        if (sim.entities[i].kind == EntityKind::Sheep) indices.push_back(i);
+    }
+    return indices;
+}
+
+void set_sheep(Sim& sim, std::size_t index, double x, double vx,
+               uint8_t state = SHEEP_STATE_WALKING,
+               double age = EligibleAge,
+               double stateTimer = LongTimer) {
+    Entity& e = sim.entities[index];
+    e.x = x;
+    e.vx = vx;
+    e.state = state;
+    e.age = age;
+    e.stateTimer = stateTimer;
+}
+
+std::vector<std::size_t> prepare_two_sheep(Sim& sim, double gap = 40.0,
+                                           double ageA = EligibleAge,
+                                           double ageB = EligibleAge) {
+    std::vector<std::size_t> indices = sheep_indices(sim);
+    REQUIRE(indices.size() >= 2);
+
+    set_sheep(sim, indices[0], 500.0, -20.0, SHEEP_STATE_WALKING, ageA);
+    set_sheep(sim, indices[1], 500.0 + gap, 18.0, SHEEP_STATE_WALKING, ageB);
+    for (std::size_t n = 2; n < indices.size(); ++n) {
+        set_sheep(sim, indices[n], 1000.0 + 150.0 * static_cast<double>(n), 16.0);
+    }
+    return indices;
+}
+
+int advance_side_past_sheep_generation(Prng& side) {
+    const double countDraw = prng_uniform(side, SHEEP_COUNT_MIN, SHEEP_COUNT_MAX + 1);
+    int expectedCount = static_cast<int>(std::floor(countDraw));
+    if (expectedCount < SHEEP_COUNT_MIN) expectedCount = SHEEP_COUNT_MIN;
+    if (expectedCount > SHEEP_COUNT_MAX) expectedCount = SHEEP_COUNT_MAX;
+
+    for (int i = 0; i < expectedCount; ++i) {
+        const double margin = SHEEP_BODY_RADIUS + 8.0;
+        (void)prng_uniform(side, margin, Monitor1920 - margin);
+        (void)prng_uniform(side, SHEEP_WALK_SPEED_MIN, SHEEP_WALK_SPEED_MAX);
+        (void)prng_uniform(side, 0.0, 1.0);
+        (void)prng_next_u32(side);
+        (void)prng_uniform(side, SHEEP_WALK_DURATION_MIN, SHEEP_WALK_DURATION_MAX);
+        (void)prng_index(side, static_cast<uint32_t>(sizeof(SHEEP_NAME_POOL) / sizeof(SHEEP_NAME_POOL[0])));
+    }
+    return expectedCount;
+}
+
+int count_sheep_in_state(const Sim& sim, uint8_t state) {
+    return static_cast<int>(std::count_if(sim.entities.begin(), sim.entities.end(),
+        [state](const Entity& e) {
+            return e.kind == EntityKind::Sheep && e.state == state;
+        }));
+}
+
+} // namespace
+
+TEST_CASE("Sheep greeting constants are pinned to spec values", "[sheep][greeting][constants]") {
+    REQUIRE(SHEEP_STATE_GREETING       == 5);
+    REQUIRE(SHEEP_GREET_RADIUS         == Approx(50.0));
+    REQUIRE(SHEEP_GREET_DURATION_MIN   == Approx(1.6));
+    REQUIRE(SHEEP_GREET_DURATION_MAX   == Approx(2.8));
+    REQUIRE(SHEEP_GREET_MIN_AGE        == Approx(1.5));
+    REQUIRE(SHEEP_GREET_HEAD_BOB_FREQ  == Approx(4.5));
+    REQUIRE(SHEEP_GREET_HEAD_BOB_AMP   == Approx(0.7));
+}
+
+TEST_CASE("Sheep curious constants are pinned to spec values", "[sheep][curious][constants]") {
+    REQUIRE(SHEEP_CURIOUS_RADIUS        == Approx(80.0));
+    REQUIRE(SHEEP_CURIOUS_HEAD_TURN_MAX == Approx(0.55));
+}
+
+
+TEST_CASE("Eligible nearby sheep enter Greeting facing each other", "[sheep][greeting]") {
+    Sim sim = build_sheep_sim();
+    const std::vector<std::size_t> indices = prepare_two_sheep(sim);
+
+    sim_tick_entities(sim, 0.016);
+
+    const Entity& a = sim.entities[indices[0]];
+    const Entity& b = sim.entities[indices[1]];
+    REQUIRE(a.state == SHEEP_STATE_GREETING);
+    REQUIRE(b.state == SHEEP_STATE_GREETING);
+    REQUIRE(a.stateTimer >= SHEEP_GREET_DURATION_MIN);
+    REQUIRE(a.stateTimer <= SHEEP_GREET_DURATION_MAX);
+    REQUIRE(a.stateTimer == Approx(b.stateTimer));
+    REQUIRE(a.vx > 0.0);
+    REQUIRE(b.vx < 0.0);
+}
+
+TEST_CASE("Far apart eligible sheep do not greet", "[sheep][greeting]") {
+    Sim sim = build_sheep_sim();
+    const std::vector<std::size_t> indices = prepare_two_sheep(sim, 200.0);
+
+    for (int i = 0; i < 3; ++i) sim_tick_entities(sim, 0.016);
+
+    REQUIRE(sim.entities[indices[0]].state == SHEEP_STATE_WALKING);
+    REQUIRE(sim.entities[indices[1]].state == SHEEP_STATE_WALKING);
+}
+
+TEST_CASE("Sheep under greeting minimum age do not greet", "[sheep][greeting]") {
+    Sim sim = build_sheep_sim();
+    const std::vector<std::size_t> indices = prepare_two_sheep(sim, 40.0, 0.5, EligibleAge);
+
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(sim.entities[indices[0]].state == SHEEP_STATE_WALKING);
+    REQUIRE(sim.entities[indices[1]].state == SHEEP_STATE_WALKING);
+}
+
+TEST_CASE("Sleeping hopping and greeting sheep are not greeting-eligible", "[sheep][greeting]") {
+    const uint8_t blockedStates[] = {
+        SHEEP_STATE_SLEEPING,
+        SHEEP_STATE_HOPPING,
+        SHEEP_STATE_GREETING,
+    };
+
+    for (uint8_t blockedState : blockedStates) {
+        Sim sim = build_sheep_sim();
+        const std::vector<std::size_t> indices = prepare_two_sheep(sim);
+        set_sheep(sim, indices[0], 500.0, -20.0, blockedState, EligibleAge);
+
+        sim_tick_entities(sim, 0.016);
+
+        REQUIRE(sim.entities[indices[0]].state == blockedState);
+        REQUIRE(sim.entities[indices[1]].state == SHEEP_STATE_WALKING);
+    }
+}
+
+TEST_CASE("Greeting expiry returns sheep to Walking with vx flipped", "[sheep][greeting]") {
+    Sim sim = build_sheep_sim();
+    const std::vector<std::size_t> indices = prepare_two_sheep(sim);
+
+    sim_tick_entities(sim, 0.016);
+    const double duration = sim.entities[indices[0]].stateTimer;
+    const double aGreetingVx = sim.entities[indices[0]].vx;
+    const double bGreetingVx = sim.entities[indices[1]].vx;
+
+    sim_tick_entities(sim, duration + 0.01);
+
+    REQUIRE(sim.entities[indices[0]].state == SHEEP_STATE_WALKING);
+    REQUIRE(sim.entities[indices[1]].state == SHEEP_STATE_WALKING);
+    REQUIRE(sim.entities[indices[0]].vx == Approx(-aGreetingVx));
+    REQUIRE(sim.entities[indices[1]].vx == Approx(-bGreetingVx));
+}
+
+TEST_CASE("Greeting trigger consumes one PRNG draw per pair", "[sheep][greeting][prng]") {
+    Prng side;
+    prng_init(side, CANONICAL_TEST_SEED ^ CRITTER_PRNG_SALT);
+
+    Sim sim = build_sheep_sim();
+    const int expectedCount = advance_side_past_sheep_generation(side);
+    REQUIRE(static_cast<int>(sheep_indices(sim).size()) == expectedCount);
+    const std::vector<std::size_t> indices = prepare_two_sheep(sim);
+
+    const double expectedDuration = prng_uniform(side,
+                                                 SHEEP_GREET_DURATION_MIN,
+                                                 SHEEP_GREET_DURATION_MAX);
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(sim.entities[indices[0]].stateTimer == Approx(expectedDuration));
+    REQUIRE(sim.entities[indices[1]].stateTimer == Approx(expectedDuration));
+}
+
+TEST_CASE("Single sheep cannot enter Greeting", "[sheep][greeting]") {
+    Sim sim = build_sheep_sim();
+    sim.currentScene = Scene::Desert;
+    REQUIRE(sim.entities.size() >= 1);
+    sim.entities.erase(sim.entities.begin() + 1, sim.entities.end());
+    set_sheep(sim, 0, 500.0, 20.0);
+
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(sim.entities.size() == 1);
+    REQUIRE(sim.entities[0].state == SHEEP_STATE_WALKING);
+}
+
+TEST_CASE("Three sheep cluster greets only the first encountered pair", "[sheep][greeting]") {
+    Sim sim = build_sheep_sim();
+    std::vector<std::size_t> indices = sheep_indices(sim);
+    REQUIRE(indices.size() >= 2);
+    if (indices.size() < 3) {
+        sim.entities.push_back(sim.entities[indices[1]]);
+        indices = sheep_indices(sim);
+    }
+    REQUIRE(indices.size() >= 3);
+
+    set_sheep(sim, indices[0], 500.0, -20.0);
+    set_sheep(sim, indices[1], 540.0, 18.0);
+    set_sheep(sim, indices[2], 580.0, 16.0);
+
+    sim_tick_entities(sim, 0.016);
+
+    REQUIRE(sim.entities[indices[0]].state == SHEEP_STATE_GREETING);
+    REQUIRE(sim.entities[indices[1]].state == SHEEP_STATE_GREETING);
+    REQUIRE(sim.entities[indices[2]].state == SHEEP_STATE_WALKING);
+    REQUIRE(count_sheep_in_state(sim, SHEEP_STATE_GREETING) == 2);
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/snapshot_data.h

@@ -0,0 +1,73 @@
+// snapshot_data.h
+//
+// Canonical snapshot values for conformance tests. Generated by
+// snapshot_gen.cpp against the Native implementation; the Win2D test
+// project shares these exact same values to prove cross-impl parity.
+//
+// To regenerate after changing the spec:
+//   cd tests/DesktopGrass.Native.Tests
+//   cl /nologo /std:c++17 /EHsc /O2 /I../../src/DesktopGrass.Native/src \
+//      /Fe:snapshot_gen.exe snapshot_gen.cpp ../../src/DesktopGrass.Native/src/Sim.cpp
+//   ./snapshot_gen.exe > snapshot_data_generated.h
+// Then copy the contents of snapshot_data_generated.h into this file.
+
+#pragma once
+
+#include <cstdint>
+#include <cstddef>
+
+namespace desktopgrass::test {
+
+// canonical PRNG snapshot (seed = 0x6B6173746F)
+constexpr uint64_t CANONICAL_PRNG_SNAPSHOT[16] = {
+    0x3C3A8D4BF44D4757ull,
+    0xC5036418082CE819ull,
+    0x637C39DC81179789ull,
+    0xA8D438AF7ACD7AE6ull,
+    0x872C242C0B1C9993ull,
+    0xEFA4F8384FDEA460ull,
+    0x1C028EE81E340128ull,
+    0x292DB46E8579232Aull,
+    0xD68F60B495865BECull,
+    0xB92C6D6C0EF02C5Bull,
+    0xEA3E31B01AEBBAC3ull,
+    0x69414C59CD84BD76ull,
+    0x824EF03EDB86298Cull,
+    0x2EC0BC0D0F34C6DFull,
+    0x06931E51B1E4F892ull,
+    0x51E8736B5F6D55E3ull,
+};
+
+// blade count: 321
+constexpr size_t CANONICAL_BLADE_COUNT = 321;
+
+// first 10 blades (baseX, height, thickness, hue, swayPhaseOffset, stiffness, isFlower, flowerHeadColorIdx, flowerHeadRadius, heightBonus)
+struct SnapshotBlade { double baseX, height, thickness; uint8_t hue; double sway, stiffness; bool isFlower; uint8_t flowerHeadColorIdx; double flowerHeadRadius, heightBonus; };
+constexpr SnapshotBlade CANONICAL_FIRST_10[10] = {
+    { 4.941073726820111, 24.469991818248864, 1.5829214329729786, 3, 3.3176304956845826, 0.97444439458772458, false, 0, 0, 1 },
+    { 9.3787298687475591, 9.8604876018392638, 2.2571879063910156, 4, 5.7491868538687054, 0.76446104886036426, false, 0, 0, 1 },
+    { 15.414797889934666, 10.383081509132303, 1.0385235237289103, 1, 3.002564694512488, 0.80184457223353733, true, 5, 1.9856510266114094, 1.2028967677469276 },
+    { 19.593121666328006, 27.357762722959727, 1.0339384653459984, 3, 1.6105552667895404, 0.81282211516340619, false, 0, 0, 1 },
+    { 24.583549065022112, 10.405811371734785, 1.3631340217308754, 3, 6.0791471337675995, 0.85778838989075124, false, 0, 0, 1 },
+    { 30.469280325562636, 14.64969214497285, 2.1029229162066789, 4, 1.369186973739968, 0.64921394446231895, false, 0, 0, 1 },
+    { 36.151633528778135, 24.905416507570557, 1.681128965493375, 5, 1.0984313545668589, 0.61705905497643643, false, 0, 0, 1 },
+    { 41.240173248804979, 21.090216438210287, 2.4112504781311586, 1, 2.5650668705987827, 0.80856258993385732, false, 0, 0, 1 },
+    { 45.909481179288093, 25.779836864342794, 1.9217430631389112, 3, 4.5760223476063198, 0.6897456846181147, false, 0, 0, 1 },
+    { 51.704527631340518, 7.0226866871355051, 2.0844748317130479, 5, 0.35993160065393376, 0.95409362721021629, false, 0, 0, 1 },
+};
+
+// last 10 blades
+constexpr SnapshotBlade CANONICAL_LAST_10[10] = {
+    { 1862.1862973905477, 12.711608036449295, 1.012073444534392, 3, 2.3651694770128948, 0.87280041193860214, false, 0, 0, 1 },
+    { 1869.0137044788548, 29.295061932038202, 1.8599729032248227, 5, 0.93125378903474243, 0.77711311572472863, false, 0, 0, 1 },
+    { 1876.3989600185221, 16.412749219937503, 2.3707904389430361, 4, 6.2236497795954646, 0.69830242079702853, false, 0, 0, 1 },
+    { 1883.2648022027838, 27.079136980574535, 1.3818519218724266, 1, 5.6607957368262252, 0.64471754349581489, false, 0, 0, 1 },
+    { 1889.9657219661015, 6.5120673117922729, 1.3927977522226092, 0, 1.0400004070684932, 0.65011504476310344, false, 0, 0, 1 },
+    { 1897.0421995171516, 22.778199667770664, 1.6103911154185315, 3, 5.4418514925265704, 0.6792514093313039, false, 0, 0, 1 },
+    { 1902.4342767348269, 14.612095947624056, 2.4718071777795467, 4, 5.8520526497642198, 0.91196804564197653, false, 0, 0, 1 },
+    { 1907.2058102690753, 11.469067214809311, 1.0067274803347863, 1, 3.1644688274678971, 0.97325380897540192, false, 0, 0, 1 },
+    { 1911.3865054893965, 26.080515240165873, 2.0193479120917956, 2, 3.350989422282157, 0.72097617434818306, false, 0, 0, 1 },
+    { 1915.6595838732392, 8.7174302729300273, 1.7257363895237519, 3, 2.9693994932808887, 0.74923939092464364, false, 0, 0, 1 },
+};
+
+} // namespace desktopgrass::test

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/sway_tests.cpp

@@ -0,0 +1,139 @@
+// sway_tests.cpp
+//
+// Sway physics tests (architecture.md §6).
+
+#include "../third_party/catch2/catch.hpp"
+#include "Sim.h"
+
+#include <cmath>
+
+using namespace desktopgrass;
+
+namespace {
+
+constexpr double kPi = 3.14159265358979323846;
+
+Blade make_blade(double phase, double stiffness) {
+    Blade b{};
+    b.baseX            = 0.0;
+    b.height           = 20.0;
+    b.thickness        = 1.5;
+    b.hue              = 0;
+    b.swayPhaseOffset  = phase;
+    b.stiffness        = stiffness;
+    b.cutHeight        = 1.0;
+    b.gustVelocity     = 0.0;
+    b.cutAnimStart     = -1.0;
+    b.cutInitialHeight = 1.0;
+    return b;
+}
+
+} // anonymous
+
+TEST_CASE("sway phase advances linearly with globalTime", "[sway]") {
+    Blade b = make_blade(0.0, 1.0);
+    update_blade_dynamics(b, 0.0, 0.016);
+    const double leanT0 = b.effectiveLean;
+
+    // After one full BASE_SWAY_SPEED period (6 sec) the lean returns to ~same.
+    update_blade_dynamics(b, (2.0 * kPi) / BASE_SWAY_SPEED, 0.016);
+    REQUIRE(b.effectiveLean == Approx(leanT0).margin(1e-9));
+}
+
+TEST_CASE("sway lean stays bounded by BASE_AMPLITUDE * stiffness", "[sway]") {
+    Blade b = make_blade(0.0, 1.0);
+    double maxAbs = 0.0;
+    // Sample one full period at fine granularity.
+    for (double t = 0.0; t < (2.0 * kPi) / BASE_SWAY_SPEED; t += 0.001) {
+        update_blade_dynamics(b, t, 0.001);
+        maxAbs = std::max(maxAbs, std::fabs(b.effectiveLean));
+    }
+    REQUIRE(maxAbs <= BASE_AMPLITUDE + 1e-9);
+    REQUIRE(maxAbs >= BASE_AMPLITUDE * 0.99);
+}
+
+TEST_CASE("stiffness scales sway amplitude", "[sway]") {
+    Blade soft = make_blade(0.0, 0.6);
+    Blade hard = make_blade(0.0, 1.0);
+
+    double softMax = 0.0, hardMax = 0.0;
+    for (double t = 0.0; t < (2.0 * kPi) / BASE_SWAY_SPEED; t += 0.001) {
+        update_blade_dynamics(soft, t, 0.001);
+        update_blade_dynamics(hard, t, 0.001);
+        softMax = std::max(softMax, std::fabs(soft.effectiveLean));
+        hardMax = std::max(hardMax, std::fabs(hard.effectiveLean));
+    }
+
+    REQUIRE(softMax <  hardMax);
+    REQUIRE(softMax == Approx(hardMax * 0.6).margin(1e-3));
+}
+
+TEST_CASE("swayAmplitude scale multiplies the lean", "[sway]") {
+    // At the same time/phase, ampScale=2.0 doubles the lean; ampScale=0 zeroes it.
+    Blade base = make_blade(0.3, 1.0);
+    Blade dbl  = make_blade(0.3, 1.0);
+    Blade zero = make_blade(0.3, 1.0);
+    const double t = 1.234;
+    update_blade_dynamics(base, t, 0.016, 1.0, 1.0);
+    update_blade_dynamics(dbl,  t, 0.016, 1.0, 2.0);
+    update_blade_dynamics(zero, t, 0.016, 1.0, 0.0);
+    REQUIRE(dbl.effectiveLean  == Approx(2.0 * base.effectiveLean).margin(1e-12));
+    REQUIRE(zero.effectiveLean == Approx(0.0).margin(1e-12));
+}
+
+TEST_CASE("swaySpeed scale stretches the phase advance", "[sway]") {
+    // speedScale=2.0 at time t equals the default at time 2t (pure phase scaling).
+    Blade fast = make_blade(0.1, 1.0);
+    Blade slow = make_blade(0.1, 1.0);
+    const double t = 0.9;
+    update_blade_dynamics(fast, t,       0.016, 2.0, 1.0);
+    update_blade_dynamics(slow, 2.0 * t, 0.016, 1.0, 1.0);
+    REQUIRE(fast.effectiveLean == Approx(slow.effectiveLean).margin(1e-12));
+}
+
+TEST_CASE("sim_tick applies the Sim sway scales to blades", "[sway]") {
+    // Proves the knobs are actually wired through the per-frame tick, not just
+    // the standalone helper: a sim with swayAmpScale=0 produces zero base lean.
+    Sim sim = sim_init(CANONICAL_TEST_SEED, 1920.0, DEFAULT_DENSITY);
+    sim.swayAmpScale   = 0.0;
+    sim.swaySpeedScale = 1.0;
+    sim_tick(sim, 0.5, nullptr, 0);
+    for (const Blade& b : sim.blades) {
+        // No ambient gust fired (gustVelocity stays 0), so effectiveLean is pure
+        // base lean, which ampScale=0 must flatten to 0.
+        REQUIRE(b.gustVelocity == Approx(0.0).margin(1e-12));
+        REQUIRE(b.effectiveLean == Approx(0.0).margin(1e-12));
+    }
+}
+
+TEST_CASE("phase offset shifts the sine wave", "[sway]") {
+    Blade a = make_blade(0.0,        1.0);
+    Blade b = make_blade(kPi / 2.0, 1.0);
+
+    update_blade_dynamics(a, 0.0, 0.001);
+    update_blade_dynamics(b, 0.0, 0.001);
+
+    // At t=0 with stiffness=1: a -> sin(0)*6 = 0; b -> sin(π/2)*6 = 6.
+    REQUIRE(a.effectiveLean == Approx(0.0).margin(1e-9));
+    REQUIRE(b.effectiveLean == Approx(BASE_AMPLITUDE).margin(1e-9));
+}
+
+TEST_CASE("gust velocity decays exponentially with dt", "[sway]") {
+    Blade b = make_blade(0.0, 1.0);
+    b.gustVelocity = 10.0;
+
+    // After 1 second, expect gustVelocity ≈ 10 * exp(-2.5).
+    update_blade_dynamics(b, 0.0, 1.0);
+    REQUIRE(b.gustVelocity == Approx(10.0 * std::exp(-DECAY_RATE * 1.0)).margin(1e-9));
+}
+
+TEST_CASE("gust velocity contributes to effective lean", "[sway]") {
+    Blade b = make_blade(0.0, 1.0);
+    b.gustVelocity = 2.0;
+
+    // tiny dt so decay is negligible
+    update_blade_dynamics(b, 0.0, 1e-6);
+    const double expectedFromGust = 2.0 * GUST_TO_LEAN_FACTOR;
+    // At t=0 sway contribution is sin(0)=0; only gust remains.
+    REQUIRE(b.effectiveLean == Approx(expectedFromGust).margin(1e-3));
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/src/winter_tests.cpp

@@ -0,0 +1,463 @@
+#include "catch.hpp"
+
+#include "Sim.h"
+#include "Constants.h"
+#include "snapshot_data.h"
+
+#include <cstddef>
+#include <cmath>
+#include <algorithm>
+#include <array>
+#include <limits>
+#include <vector>
+
+using namespace desktopgrass;
+using namespace desktopgrass::test;
+
+namespace {
+constexpr double kTwoPi = 6.28318530717958647692;
+
+Sim MakeWinterTestSim() {
+    return sim_init(CANONICAL_TEST_SEED, 1920.0, 1.0);
+}
+
+void TickUntilFirstSnowflake(Sim& sim) {
+    for (int i = 0; i < 10000 && sim.entities.empty(); ++i) {
+        sim_tick(sim, 0.01, nullptr, 0);
+    }
+    REQUIRE_FALSE(sim.entities.empty());
+}
+}
+
+TEST_CASE("Winter constants are pinned", "[winter][constants]") {
+    REQUIRE(SNOWFLAKE_EMIT_RATE_PER_1920DIP == Approx(8.0));
+    REQUIRE(SNOWFLAKE_FALL_SPEED_MIN == Approx(20.0));
+    REQUIRE(SNOWFLAKE_FALL_SPEED_MAX == Approx(40.0));
+    REQUIRE(SNOWFLAKE_SIZE_MIN == Approx(1.5));
+    REQUIRE(SNOWFLAKE_SWAY_AMPLITUDE == Approx(10.0));
+    REQUIRE(SNOWFLAKE_PRNG_SALT == 0xC0FFEE1CECAFEBABull);
+    REQUIRE(SNOW_TIP_RADIUS_FACTOR == Approx(1.25));
+    REQUIRE(SNOW_TIP_COLOR == 0xFFFFFFFFu);
+}
+
+TEST_CASE("Winter blade cull is deterministic and ~25%", "[winter][cull]") {
+    // Pinned bitmask for indices 0..31 — must match the Win2D renderer exactly so
+    // both impls thin the same blades. '1' == culled (skipped in Winter).
+    const char* kExpected = "10100111000100000000000010000000";
+    for (uint32_t i = 0; i < 32; ++i) {
+        const bool expected = kExpected[i] == '1';
+        REQUIRE(winter_blade_culled(i) == expected);
+    }
+
+    REQUIRE(WINTER_CULL_MASK == 3u);
+
+    int culled = 0;
+    for (uint32_t i = 0; i < 2500; ++i) {
+        if (winter_blade_culled(i)) ++culled;
+    }
+    REQUIRE(culled == 624); // 24.96% of 2500 — effectively the target 25%
+}
+
+TEST_CASE("SetScene Winter initializes snowflake scheduler", "[winter][scene]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    REQUIRE(sim.nextSnowflakeSpawnTime > sim.globalTime);
+    REQUIRE(sim.nextSnowflakeSpawnTime < 100.0);
+}
+
+TEST_CASE("First winter snowflake emits on scheduled tick", "[winter][entities]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    TickUntilFirstSnowflake(sim);
+
+    REQUIRE(sim.entities.size() == 1);
+    REQUIRE(sim.entities[0].kind == EntityKind::Snowflake);
+}
+
+TEST_CASE("First winter snowflake matches spec-derived PRNG snapshot", "[winter][entities][snapshot]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    TickUntilFirstSnowflake(sim);
+    REQUIRE(sim.entities.size() == 1);
+
+    Prng expected{};
+    prng_init(expected, CANONICAL_TEST_SEED ^ SNOWFLAKE_PRNG_SALT);
+    const double lambda = SNOWFLAKE_EMIT_RATE_PER_1920DIP * sim.monitorWidth / 1920.0;
+    const double firstInterval = prng_exponential(expected, lambda);
+    const double expectedSize = prng_uniform(expected, SNOWFLAKE_SIZE_MIN, SNOWFLAKE_SIZE_MAX);
+    const double expectedX = prng_uniform(expected, -20.0, sim.monitorWidth + 20.0);
+    const double expectedFallSpeed = prng_uniform(expected, SNOWFLAKE_FALL_SPEED_MIN, SNOWFLAKE_FALL_SPEED_MAX);
+    const double expectedRotation = prng_uniform(expected, 0.0, kTwoPi);
+    const double expectedRotationSpeed = prng_uniform(expected, -1.5, 1.5);
+    const uint32_t expectedSeed = prng_next_u32(expected);
+    const double nextInterval = prng_exponential(expected, lambda);
+
+    const Entity& e = sim.entities[0];
+    REQUIRE(e.size == Approx(expectedSize).margin(1e-12));
+    REQUIRE(e.x == Approx(expectedX).margin(1e-12));
+    REQUIRE(e.vy == Approx(expectedFallSpeed).margin(1e-12));
+    REQUIRE(e.rotation == Approx(expectedRotation).margin(1e-12));
+    REQUIRE(e.rotationSpeed == Approx(expectedRotationSpeed).margin(1e-12));
+    REQUIRE(e.seed == expectedSeed);
+    REQUIRE(sim.nextSnowflakeSpawnTime == Approx(firstInterval + nextInterval).margin(1e-12));
+}
+
+TEST_CASE("Snowflake sway velocity wobbles from seed phase", "[winter][entities]") {
+    Sim sim = MakeWinterTestSim();
+    sim.currentScene = Scene::Desert;
+    Entity e{};
+    e.kind = EntityKind::Snowflake;
+    e.seed = 0;
+    e.age = 0.0;
+    e.lifetime = 100.0;
+    sim.entities.push_back(e);
+
+    sim_tick_entities(sim, 0.0);
+
+    const double expectedVx = SNOWFLAKE_SWAY_AMPLITUDE * SNOWFLAKE_SWAY_FREQUENCY * kTwoPi * std::cos(0.0);
+    REQUIRE(sim.entities.size() == 1);
+    REQUIRE(sim.entities[0].vx == Approx(expectedVx).margin(1e-12));
+}
+
+TEST_CASE("Snowflakes are culled after lifetime", "[winter][entities]") {
+    Sim sim = MakeWinterTestSim();
+    sim.currentScene = Scene::Desert;
+    Entity e{};
+    e.kind = EntityKind::Snowflake;
+    e.lifetime = 1.0;
+    e.age = 0.9;
+    sim.entities.push_back(e);
+
+    sim_tick_entities(sim, 0.2);
+
+    REQUIRE(sim.entities.empty());
+}
+
+TEST_CASE("Snowflakes are culled below ground line", "[winter][entities]") {
+    Sim sim = MakeWinterTestSim();
+    sim.currentScene = Scene::Desert;
+    Entity e{};
+    e.kind = EntityKind::Snowflake;
+    e.y = sim.windowHeight + 5.0;
+    e.lifetime = 100.0;
+    sim.entities.push_back(e);
+
+    sim_tick_entities(sim, 0.0);
+
+    REQUIRE(sim.entities.empty());
+}
+
+TEST_CASE("Winter snowflake emitter honors max entity cap", "[winter][entities]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+    sim.nextSnowflakeSpawnTime = sim.globalTime;
+    for (int i = 0; i < MAX_ENTITIES_PER_MONITOR; ++i) {
+        Entity e{};
+        e.kind = EntityKind::Snowflake;
+        e.lifetime = 100.0;
+        sim.entities.push_back(e);
+    }
+
+    sim_tick_entities(sim, 0.0);
+
+    REQUIRE(sim.entities.size() <= static_cast<std::size_t>(MAX_ENTITIES_PER_MONITOR));
+    REQUIRE(sim.entities.size() == static_cast<std::size_t>(MAX_ENTITIES_PER_MONITOR));
+}
+
+TEST_CASE("Winter scene does not perturb first-blade snapshot", "[winter][snapshot]") {
+    Sim sim = MakeWinterTestSim();
+    REQUIRE(sim.blades[0].baseX == Approx(CANONICAL_FIRST_10[0].baseX).margin(1e-12));
+
+    sim_set_scene(sim, Scene::Winter);
+    REQUIRE(sim.blades[0].baseX == Approx(CANONICAL_FIRST_10[0].baseX).margin(1e-12));
+
+    sim_set_scene(sim, Scene::Grass);
+    REQUIRE(sim.blades[0].baseX == Approx(CANONICAL_FIRST_10[0].baseX).margin(1e-12));
+}
+
+TEST_CASE("Snowflakes do not emit in non-winter scenes", "[winter][entities][scene]") {
+    Sim sim = MakeWinterTestSim();
+
+    sim_set_scene(sim, Scene::Grass);
+    sim.nextSnowflakeSpawnTime = 0.0;
+    sim_tick(sim, 2.0, nullptr, 0);
+    REQUIRE(std::none_of(sim.entities.begin(), sim.entities.end(),
+        [](const Entity& e) { return e.kind == EntityKind::Snowflake; }));
+
+    sim_set_scene(sim, Scene::Desert);
+    sim.entities.clear();
+    sim.nextSnowflakeSpawnTime = 0.0;
+    sim_tick(sim, 2.0, nullptr, 0);
+    REQUIRE(sim.entities.empty());
+}
+
+namespace {
+int count_snow_puffs(const Sim& sim) {
+    int n = 0;
+    for (const Entity& e : sim.entities)
+        if (e.kind == EntityKind::SnowPuff) ++n;
+    return n;
+}
+
+InputEvent WinterClick(const Sim& sim, double x) {
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x    = x;
+    ev.y    = sim.windowHeight - 5.0;
+    ev.time = sim.globalTime;
+    return ev;
+}
+}
+
+TEST_CASE("Snow puff constants are pinned", "[winter][puff][constants]") {
+    REQUIRE(SNOW_PUFF_COUNT_MIN == 9);
+    REQUIRE(SNOW_PUFF_COUNT_MAX == 16);
+    REQUIRE(SNOW_PUFF_SIZE_MIN == Approx(3.5));
+    REQUIRE(SNOW_PUFF_SIZE_MAX == Approx(8.0));
+    REQUIRE(SNOW_PUFF_GRAVITY == Approx(150.0));
+    REQUIRE(SNOW_PUFF_DRAG == Approx(1.6));
+    REQUIRE(SNOW_PUFF_SPREAD_RAD == Approx(1.25));
+    REQUIRE(SNOW_PUFF_PRNG_SALT == 0x5503FF1E5503FF1Eull);
+}
+
+TEST_CASE("Clicking the winter snowbank sheds a snow puff burst", "[winter][puff]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    InputEvent ev = WinterClick(sim, 400.0);
+    sim_apply_click(sim, ev);
+
+    const int puffs = count_snow_puffs(sim);
+    REQUIRE(puffs >= SNOW_PUFF_COUNT_MIN);
+    REQUIRE(puffs <= SNOW_PUFF_COUNT_MAX);
+
+    // Every puff launches upward (y is screen-down, so up is negative vy) and
+    // spawns at or above the ground line.
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::SnowPuff) continue;
+        REQUIRE(e.vy < 0.0);
+        REQUIRE(e.y <= sim.windowHeight + 1e-9);
+    }
+}
+
+TEST_CASE("Snow puff only fires in Winter", "[winter][puff][scene]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Grass);
+
+    InputEvent ev = WinterClick(sim, 400.0);
+    sim_apply_click(sim, ev);
+
+    REQUIRE(count_snow_puffs(sim) == 0);
+}
+
+TEST_CASE("A non-finite click sheds no snow puff", "[winter][puff][guard]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    InputEvent ev{};
+    ev.type = EventType::Click;
+    ev.x    = std::numeric_limits<double>::quiet_NaN();
+    ev.y    = sim.windowHeight - 5.0;
+    ev.time = sim.globalTime;
+    sim_apply_click(sim, ev);
+
+    REQUIRE(count_snow_puffs(sim) == 0);
+}
+
+TEST_CASE("Snow puff burst rises then settles and is culled", "[winter][puff]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    InputEvent ev = WinterClick(sim, 400.0);
+    sim_apply_click(sim, ev);
+    REQUIRE(count_snow_puffs(sim) > 0);
+
+    // 4 s easily exceeds SNOW_PUFF_LIFETIME_MAX (1.8 s); every puff should be
+    // culled (lifetime expiry and/or falling back below the ground line).
+    for (int i = 0; i < 200; ++i) sim_tick_entities(sim, 0.02);
+    REQUIRE(count_snow_puffs(sim) == 0);
+}
+
+TEST_CASE("Snow puff draw order matches a side PRNG stream", "[winter][puff][prng]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    InputEvent ev = WinterClick(sim, 300.0);
+    sim_apply_click(sim, ev);
+
+    Prng side{};
+    prng_init(side, CANONICAL_TEST_SEED ^ SNOW_PUFF_PRNG_SALT);
+    const int expectedCount = SNOW_PUFF_COUNT_MIN
+        + static_cast<int>(prng_index(side, SNOW_PUFF_COUNT_MAX - SNOW_PUFF_COUNT_MIN + 1));
+    REQUIRE(count_snow_puffs(sim) == expectedCount);
+
+    // The first locked draw inside make_snow_puff is `size`.
+    const double expectedSize = prng_uniform(side, SNOW_PUFF_SIZE_MIN, SNOW_PUFF_SIZE_MAX);
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::SnowPuff) continue;
+        REQUIRE(e.size == Approx(expectedSize).margin(1e-12));
+        break;
+    }
+}
+
+TEST_CASE("Snow puff salt is unique among winter PRNG salts", "[winter][puff][prng]") {
+    const std::array<uint64_t, 15> otherSalts = {
+        REGROW_PRNG_SALT, FLOWER_PRNG_SALT, MUSHROOM_PRNG_SALT,
+        AMBIENT_GUST_PRNG_SALT, CACTUS_PRNG_SALT, TUMBLEWEED_PRNG_SALT,
+        CRITTER_PRNG_SALT, BUTTERFLY_PRNG_SALT, FIREFLY_PRNG_SALT,
+        BIRD_FLYBY_PRNG_SALT, SNOWFLAKE_PRNG_SALT,
+        PINE_PRNG_SALT, LEAF_PUFF_PRNG_SALT, SNOW_DRIFT_PRNG_SALT,
+    };
+    for (uint64_t s : otherSalts) {
+        REQUIRE(SNOW_PUFF_PRNG_SALT != s);
+    }
+}
+
+// ---------------------------------------------------------------------------
+// §21.1 snow drift (cursor-move spindrift)
+// ---------------------------------------------------------------------------
+
+namespace {
+// Prime the cursor baseline, then brush across at `x0`→`x1` over `dt` seconds in
+// the low snow band. Returns the velocity-carrying second event already applied.
+void WinterDrift(Sim& sim, double x0, double x1, double dt) {
+    const double y = sim.windowHeight - 5.0;
+    InputEvent prime{};
+    prime.type = EventType::Move;
+    prime.x = x0; prime.y = y; prime.time = sim.globalTime;
+    sim_apply_move(sim, prime);
+
+    InputEvent move{};
+    move.type = EventType::Move;
+    move.x = x1; move.y = y; move.time = sim.globalTime + dt;
+    sim_apply_move(sim, move);
+}
+}
+
+TEST_CASE("Snow drift constants are pinned", "[winter][drift][constants]") {
+    REQUIRE(SNOW_DRIFT_COUNT_MIN == 4);
+    REQUIRE(SNOW_DRIFT_COUNT_MAX == 8);
+    REQUIRE(SNOW_DRIFT_REACH_DIP == Approx(70.0));
+    REQUIRE(SNOW_DRIFT_MIN_SPEED == Approx(90.0));
+    REQUIRE(SNOW_DRIFT_COOLDOWN_SEC == Approx(0.12));
+    REQUIRE(SNOW_DRIFT_SIZE_SCALE == Approx(0.9));
+    REQUIRE(SNOW_DRIFT_SPEED_SCALE == Approx(0.85));
+    REQUIRE(SNOW_DRIFT_PRNG_SALT == 0x5D81F77D5D81F77Dull);
+}
+
+TEST_CASE("Brushing the cursor across the snowbank kicks up a drift wisp", "[winter][drift]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    WinterDrift(sim, 300.0, 360.0, 0.05); // 60 DIP / 0.05 s = 1200 DIP/s
+
+    const int puffs = count_snow_puffs(sim);
+    REQUIRE(puffs >= SNOW_DRIFT_COUNT_MIN);
+    REQUIRE(puffs <= SNOW_DRIFT_COUNT_MAX);
+
+    // Drift grains are smaller than a click burst and still launch upward.
+    for (const Entity& e : sim.entities) {
+        if (e.kind != EntityKind::SnowPuff) continue;
+        REQUIRE(e.vy < 0.0);
+        REQUIRE(e.size <= SNOW_PUFF_SIZE_MAX * SNOW_DRIFT_SIZE_SCALE + 1e-9);
+        // Drift puffs originate at the snow surface beneath the cursor, not at
+        // the cursor's floating height: y sits within START_RADIUS of the
+        // ground even though the cursor is 5 DIP above it.
+        const double groundY = sim.windowHeight;
+        REQUIRE(e.y <= groundY + 1e-9);
+        REQUIRE(e.y >= groundY - SNOW_PUFF_START_RADIUS - 1e-9);
+    }
+}
+
+TEST_CASE("Snow drift only fires in Winter", "[winter][drift][scene]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Grass);
+
+    WinterDrift(sim, 300.0, 360.0, 0.05);
+
+    REQUIRE(count_snow_puffs(sim) == 0);
+}
+
+TEST_CASE("A slow cursor brush kicks up no drift", "[winter][drift][gate]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    WinterDrift(sim, 300.0, 302.0, 0.05); // 2 DIP / 0.05 s = 40 DIP/s < 90
+
+    REQUIRE(count_snow_puffs(sim) == 0);
+}
+
+TEST_CASE("A high cursor brush above the snow band kicks up no drift", "[winter][drift][gate]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    // Inside the gust band but far above the low drift band near the ground.
+    const double y = sim.windowHeight - SNOW_DRIFT_REACH_DIP - 20.0;
+    InputEvent prime{};
+    prime.type = EventType::Move; prime.x = 300.0; prime.y = y; prime.time = sim.globalTime;
+    sim_apply_move(sim, prime);
+    InputEvent move{};
+    move.type = EventType::Move; move.x = 360.0; move.y = y; move.time = sim.globalTime + 0.05;
+    sim_apply_move(sim, move);
+
+    REQUIRE(count_snow_puffs(sim) == 0);
+}
+
+TEST_CASE("Snow drift respects the global cooldown", "[winter][drift][cooldown]") {
+    Sim sim = MakeWinterTestSim();
+    sim_set_scene(sim, Scene::Winter);
+
+    WinterDrift(sim, 300.0, 360.0, 0.05);
+    const int first = count_snow_puffs(sim);
+    REQUIRE(first >= SNOW_DRIFT_COUNT_MIN);
+
+    // Same frame (globalTime unchanged): a second qualifying brush is gated.
+    InputEvent again{};
+    again.type = EventType::Move;
+    again.x = 420.0; again.y = sim.windowHeight - 5.0; again.time = sim.globalTime + 0.10;
+    sim_apply_move(sim, again);
+    REQUIRE(count_snow_puffs(sim) == first);
+
+    // Advance past the cooldown: a fresh brush kicks up another wisp.
+    sim.globalTime += SNOW_DRIFT_COOLDOWN_SEC + 0.01;
+    InputEvent later{};
+    later.type = EventType::Move;
+    later.x = 480.0; later.y = sim.windowHeight - 5.0; later.time = sim.globalTime + 0.05;
+    sim_apply_move(sim, later);
+    REQUIRE(count_snow_puffs(sim) > first);
+}
+
+TEST_CASE("Snow drift moves leave the click puff stream untouched", "[winter][drift][prng]") {
+    Sim a = MakeWinterTestSim();
+    sim_set_scene(a, Scene::Winter);
+    Sim b = MakeWinterTestSim();
+    sim_set_scene(b, Scene::Winter);
+
+    // a brushes up some drift wisps first; b does not.
+    WinterDrift(a, 300.0, 360.0, 0.05);
+    const std::size_t aPreClick = a.entities.size();
+
+    // Both click identically; the click puffs must match byte-for-byte because
+    // the click stream is a separate PRNG from the drift stream.
+    InputEvent ca = WinterClick(a, 800.0);
+    sim_apply_click(a, ca);
+    InputEvent cb = WinterClick(b, 800.0);
+    sim_apply_click(b, cb);
+
+    // Collect the click puffs from each (a's are those appended after the drift).
+    std::vector<Entity> aClick(a.entities.begin() + static_cast<std::ptrdiff_t>(aPreClick), a.entities.end());
+    std::vector<Entity> bClick;
+    for (const Entity& e : b.entities)
+        if (e.kind == EntityKind::SnowPuff) bClick.push_back(e);
+
+    REQUIRE(aClick.size() == bClick.size());
+    for (std::size_t i = 0; i < aClick.size(); ++i) {
+        REQUIRE(aClick[i].size == Approx(bClick[i].size).margin(1e-12));
+        REQUIRE(aClick[i].vx == Approx(bClick[i].vx).margin(1e-12));
+        REQUIRE(aClick[i].vy == Approx(bClick[i].vy).margin(1e-12));
+        REQUIRE(aClick[i].lifetime == Approx(bClick[i].lifetime).margin(1e-12));
+    }
+}

src/modules/DesktopGrass/DesktopGrass.Native.Tests/third_party/README.md

@@ -0,0 +1,14 @@
+# DesktopGrass.Native.Tests — third_party
+
+This directory contains source vendored at known versions to keep the test
+build hermetic. None of it ships in the runtime binary.
+
+## catch2/catch.hpp
+
+[Catch2](https://github.com/catchorg/Catch2) v2.13.10 single-header
+amalgamation, vendored verbatim from the upstream release. License: Boost
+Software License 1.0. Copy lives at `catch2/catch.hpp`.
+
+We intentionally avoid pulling Catch2 from vcpkg/NuGet for this v1 — the
+single-header approach builds with `cl` out of the box and removes one
+moving piece from the test step.

src/modules/DesktopGrass/DesktopGrass.Native.Tests/vcpkg.json

@@ -0,0 +1,6 @@
+{
+  "name": "desktopgrass-native-tests",
+  "version-string": "1.0.0",
+  "description": "DesktopGrass Native test project. Catch2 single-header is vendored under third_party/catch2/ (license MIT) to keep the test build fully offline-friendly.",
+  "dependencies": []
+}

src/modules/DesktopGrass/DesktopGrass.Native/DesktopGrass.Native.rc

@@ -0,0 +1,11 @@
+// DesktopGrass.Native.rc
+//
+// Resource script: embeds the icon and the app manifest.
+
+#include <windows.h>
+#include "resource.h"
+
+IDI_APPICON  ICON  "res/icon.ico"
+IDI_TRAYICON ICON  "res/icon.ico"
+
+// Manifest binding handled by linker /MANIFESTUAC + <ApplicationManifest>.

src/modules/DesktopGrass/DesktopGrass.Native/DesktopGrass.Native.vcxproj

@@ -0,0 +1,178 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+  <ItemGroup Label="ProjectConfigurations">
+    <ProjectConfiguration Include="Debug|x64">
+      <Configuration>Debug</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|x64">
+      <Configuration>Release</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Debug|ARM64">
+      <Configuration>Debug</Configuration>
+      <Platform>ARM64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|ARM64">
+      <Configuration>Release</Configuration>
+      <Platform>ARM64</Platform>
+    </ProjectConfiguration>
+  </ItemGroup>
+
+  <PropertyGroup Label="Globals">
+    <VCProjectVersion>17.0</VCProjectVersion>
+    <ProjectGuid>{B0D4E1B0-1F5E-4C2D-9F44-DA8C3F1A2A11}</ProjectGuid>
+    <RootNamespace>DesktopGrass.Native</RootNamespace>
+    <WindowsTargetPlatformVersion>10.0</WindowsTargetPlatformVersion>
+    <ProjectName>DesktopGrass.Native</ProjectName>
+  </PropertyGroup>
+
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
+
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>true</UseDebugLibraries>
+    <PlatformToolset>v145</PlatformToolset>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>false</UseDebugLibraries>
+    <PlatformToolset>v145</PlatformToolset>
+    <WholeProgramOptimization>true</WholeProgramOptimization>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|ARM64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>true</UseDebugLibraries>
+    <PlatformToolset>v145</PlatformToolset>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|ARM64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>false</UseDebugLibraries>
+    <PlatformToolset>v145</PlatformToolset>
+    <WholeProgramOptimization>true</WholeProgramOptimization>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />
+
+  <PropertyGroup>
+    <OutDir>$(MSBuildProjectDirectory)\..\..\..\..\$(Platform)\$(Configuration)\</OutDir>
+    <IntDir>$(MSBuildProjectDirectory)\obj\$(Platform)\$(Configuration)\</IntDir>
+    <TargetName>DesktopGrass.Native</TargetName>
+    <!-- DesktopGrass.Native uses no precompiled header; opt out of the PowerToys-wide PCH default. -->
+    <UsePrecompiledHeaders>false</UsePrecompiledHeaders>
+    <!-- Self-contained leaf module: opt out of the repo-wide CppCoreCheck-as-errors (cf. FileLocksmithCLI). -->
+    <RunCodeAnalysis>false</RunCodeAnalysis>
+  </PropertyGroup>
+
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <LanguageStandard>stdcpp17</LanguageStandard>
+      <SDLCheck>true</SDLCheck>
+      <ConformanceMode>true</ConformanceMode>
+      <WarningLevel>Level3</WarningLevel>
+      <PreprocessorDefinitions>UNICODE;_UNICODE;NOMINMAX;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <MultiProcessorCompilation>true</MultiProcessorCompilation>
+      <AdditionalIncludeDirectories>$(MSBuildProjectDirectory)\src;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ClCompile>
+    <Link>
+      <SubSystem>Windows</SubSystem>
+      <GenerateDebugInformation>true</GenerateDebugInformation>
+      <AdditionalDependencies>d3d11.lib;dxgi.lib;d2d1.lib;dcomp.lib;dwrite.lib;Shcore.lib;Shell32.lib;User32.lib;Gdi32.lib;Ole32.lib;%(AdditionalDependencies)</AdditionalDependencies>
+    </Link>
+    <ResourceCompile>
+      <PreprocessorDefinitions>UNICODE;_UNICODE;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <AdditionalIncludeDirectories>$(MSBuildProjectDirectory);%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ResourceCompile>
+    <Manifest>
+      <AdditionalManifestFiles>app.manifest</AdditionalManifestFiles>
+    </Manifest>
+  </ItemDefinitionGroup>
+
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
+    <ClCompile>
+      <Optimization>Disabled</Optimization>
+      <RuntimeLibrary>MultiThreadedDebugDLL</RuntimeLibrary>
+      <PreprocessorDefinitions>_DEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+    </ClCompile>
+  </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'">
+    <ClCompile>
+      <Optimization>MaxSpeed</Optimization>
+      <RuntimeLibrary>MultiThreaded</RuntimeLibrary>
+      <PreprocessorDefinitions>NDEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <FunctionLevelLinking>true</FunctionLevelLinking>
+      <IntrinsicFunctions>true</IntrinsicFunctions>
+    </ClCompile>
+    <Link>
+      <EnableCOMDATFolding>true</EnableCOMDATFolding>
+      <OptimizeReferences>true</OptimizeReferences>
+    </Link>
+  </ItemDefinitionGroup>
+
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|ARM64'">
+    <ClCompile>
+      <Optimization>Disabled</Optimization>
+      <RuntimeLibrary>MultiThreadedDebugDLL</RuntimeLibrary>
+      <PreprocessorDefinitions>_DEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+    </ClCompile>
+  </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|ARM64'">
+    <ClCompile>
+      <Optimization>MaxSpeed</Optimization>
+      <RuntimeLibrary>MultiThreaded</RuntimeLibrary>
+      <PreprocessorDefinitions>NDEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <FunctionLevelLinking>true</FunctionLevelLinking>
+      <IntrinsicFunctions>true</IntrinsicFunctions>
+    </ClCompile>
+    <Link>
+      <EnableCOMDATFolding>true</EnableCOMDATFolding>
+      <OptimizeReferences>true</OptimizeReferences>
+    </Link>
+  </ItemDefinitionGroup>
+
+  <ItemGroup>
+    <ClCompile Include="src\main.cpp" />
+    <ClCompile Include="src\App.cpp" />
+    <ClCompile Include="src\AutoStart.cpp" />
+    <ClCompile Include="src\Benchmark.cpp" />
+    <ClCompile Include="src\Config.cpp" />
+    <ClCompile Include="src\GrassWindow.cpp" />
+    <ClCompile Include="src\Renderer.cpp" />
+    <ClCompile Include="src\MouseHook.cpp" />
+    <ClCompile Include="src\Pacing.cpp" />
+    <ClCompile Include="src\Persistence.cpp" />
+    <ClCompile Include="src\Sim.cpp" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ClInclude Include="src\App.h" />
+    <ClInclude Include="src\AutoStart.h" />
+    <ClInclude Include="src\Benchmark.h" />
+    <ClInclude Include="src\Config.h" />
+    <ClInclude Include="src\Constants.h" />
+    <ClInclude Include="src\GrassWindow.h" />
+    <ClInclude Include="src\Json.h" />
+    <ClInclude Include="src\MouseHook.h" />
+    <ClInclude Include="src\Pacing.h" />
+    <ClInclude Include="src\Persistence.h" />
+    <ClInclude Include="src\Renderer.h" />
+    <ClInclude Include="src\Sim.h" />
+    <ClInclude Include="resource.h" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ResourceCompile Include="DesktopGrass.Native.rc" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Include="app.manifest" />
+    <None Include="res\icon.ico" />
+  </ItemGroup>
+
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
+</Project>

src/modules/DesktopGrass/DesktopGrass.Native/app.manifest

@@ -0,0 +1,30 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+  <assemblyIdentity version="1.0.0.0"
+                    processorArchitecture="*"
+                    name="DesktopGrass.Native"
+                    type="win32"/>
+  <description>DesktopGrass — procedural grass on the desktop edge.</description>
+  <dependency>
+    <dependentAssembly>
+      <assemblyIdentity type="win32"
+                        name="Microsoft.Windows.Common-Controls"
+                        version="6.0.0.0"
+                        processorArchitecture="*"
+                        publicKeyToken="6595b64144ccf1df"
+                        language="*"/>
+    </dependentAssembly>
+  </dependency>
+  <application xmlns="urn:schemas-microsoft-com:asm.v3">
+    <windowsSettings>
+      <dpiAwareness xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitorV2</dpiAwareness>
+      <longPathAware xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">true</longPathAware>
+    </windowsSettings>
+  </application>
+  <compatibility xmlns="urn:schemas-microsoft-com:compatibility.v1">
+    <application>
+      <!-- Windows 10 / 11 -->
+      <supportedOS Id="{8e0f7a12-bfb3-4fe8-b9a5-48fd50a15a9a}"/>
+    </application>
+  </compatibility>
+</assembly>

src/modules/DesktopGrass/DesktopGrass.Native/resource.h

@@ -0,0 +1,6 @@
+// resource.h
+
+#pragma once
+
+#define IDI_APPICON   101
+#define IDI_TRAYICON  102

src/modules/DesktopGrass/DesktopGrass.Native/src/App.cpp

@@ -0,0 +1,577 @@
+// App.cpp
+
+#include "App.h"
+
+#include "AutoStart.h"
+#include "Constants.h"
+#include "Sim.h"
+#include "../resource.h"
+
+#include <shellscalingapi.h>
+#include <algorithm>
+#include <cstdio>
+#include <string>
+#include <utility>
+
+#pragma comment(lib, "shell32.lib")
+#pragma comment(lib, "Shcore.lib")
+#pragma comment(lib, "User32.lib")
+
+namespace desktopgrass {
+
+namespace {
+
+constexpr const wchar_t* kMsgWindowClass = L"DesktopGrass.Native.MessageWindow";
+
+// Fixed launch seed shared with the Win2D implementation so both produce
+// identical, deterministic per-monitor blade layouts.
+constexpr uint64_t kAppSeed = 0xD3C7C0F30070D511ull;
+
+// Per-monitor seed: combine the fixed app seed with the monitor's physical
+// origin so different screens get different — but stable across launches —
+// blade layouts. Mirrors Win2D App.cs exactly, including C# `(ulong)int`
+// sign-extension and unchecked uint64 multiply/wraparound semantics.
+uint64_t make_monitor_seed(const RECT& bounds) {
+    const uint64_t left = static_cast<uint64_t>(static_cast<int64_t>(bounds.left));
+    const uint64_t top  = static_cast<uint64_t>(static_cast<int64_t>(bounds.top));
+    return kAppSeed
+        ^ (left * 0xA0761D6478BD642Full)
+        ^ (top  * 0xE7037ED1A0B428DBull);
+}
+
+// EnumDisplayMonitors callback context.
+struct MonitorEnumCtx {
+    App*                app;
+    std::vector<RECT>   bounds;
+    std::vector<UINT>   dpis;
+};
+
+BOOL CALLBACK MonitorEnumProc(HMONITOR hMon, HDC, LPRECT, LPARAM lParam) {
+    auto* ctx = reinterpret_cast<MonitorEnumCtx*>(lParam);
+    MONITORINFO mi{};
+    mi.cbSize = sizeof(mi);
+    if (GetMonitorInfoW(hMon, &mi)) {
+        // Use the work area, not the full monitor rect, so the grass sits on
+        // top of the taskbar instead of being drawn behind it. On monitors with
+        // no taskbar (typical secondary displays), rcWork == rcMonitor.
+        ctx->bounds.push_back(mi.rcWork);
+        UINT xDpi = 96, yDpi = 96;
+        if (FAILED(GetDpiForMonitor(hMon, MDT_EFFECTIVE_DPI, &xDpi, &yDpi))) {
+            xDpi = 96;
+        }
+        ctx->dpis.push_back(xDpi);
+    }
+    return TRUE;
+}
+
+} // anonymous
+
+App::~App() {
+    DestroyAllGrassWindows();
+    RemoveTrayIcon();
+    if (trayMenu_) { DestroyMenu(trayMenu_); trayMenu_ = nullptr; }
+    DestroyMessageWindow();
+    uninstall_mouse_hook();
+}
+
+bool App::Initialize(HINSTANCE hInst) {
+    hInst_   = hInst;
+    config_  = config::LoadConfig();
+
+    QueryPerformanceFrequency(&qpcFreq_);
+    QueryPerformanceCounter(&qpcLast_);
+
+    hasPersistedState_ = persistence::LoadAppState(persistedState_);
+    if (hasPersistedState_) {
+        currentScene_ = persistedState_.scene;
+        currentCritter_ = persistedState_.critter;
+        currentCritterCount_ = persistedState_.critterCountOverride;
+        autoStart_ = persistedState_.autoStart;
+    }
+    if (!autostart::ReconcileWithState(autoStart_)) {
+        OutputDebugStringA("[DesktopGrass] unable to reconcile Start with Windows registry state\n");
+    }
+    lastPersistenceSaveMs_ = GetTickCount64();
+
+    if (!GrassWindow::RegisterWindowClass(hInst_)) return false;
+    if (!CreateMessageWindow())                    return false;
+    if (!CreateTrayIcon())                         return false;
+    if (!EnumerateMonitorsAndCreateWindows())      return false;
+
+    if (!install_mouse_hook(&queue_)) {
+        OutputDebugStringA("[DesktopGrass] install_mouse_hook failed\n");
+        // Non-fatal — the grass will still sway, just no gusts/cuts.
+    }
+
+    return true;
+}
+
+bool App::CreateMessageWindow() {
+    WNDCLASSEXW wc{};
+    wc.cbSize        = sizeof(wc);
+    wc.lpfnWndProc   = App::MessageWindowProc;
+    wc.hInstance     = hInst_;
+    wc.lpszClassName = kMsgWindowClass;
+
+    ATOM atom = RegisterClassExW(&wc);
+    if (atom == 0 && GetLastError() != ERROR_CLASS_ALREADY_EXISTS) {
+        return false;
+    }
+
+    msgHwnd_ = CreateWindowExW(
+        0, kMsgWindowClass, L"DesktopGrass.Msg",
+        0, 0, 0, 0, 0,
+        HWND_MESSAGE, nullptr, hInst_, this);
+    return msgHwnd_ != nullptr;
+}
+
+void App::DestroyMessageWindow() {
+    if (msgHwnd_) {
+        DestroyWindow(msgHwnd_);
+        msgHwnd_ = nullptr;
+    }
+}
+
+bool App::CreateTrayIcon() {
+    // Build the menu: Scene ▸ (radio: Grass / Desert / Winter / Autumn) | Quit.
+    // The Scene submenu is a child popup of trayMenu_; DestroyMenu is
+    // recursive so destroying trayMenu_ cleans up the submenu too.
+    trayMenu_ = CreatePopupMenu();
+    if (!trayMenu_) return false;
+    sceneSubmenu_ = CreatePopupMenu();
+    if (!sceneSubmenu_) return false;
+    AppendMenuW(sceneSubmenu_, MF_STRING, kMenuSceneGrass,  L"Grass");
+    AppendMenuW(sceneSubmenu_, MF_STRING, kMenuSceneDesert, L"Desert");
+    AppendMenuW(sceneSubmenu_, MF_STRING, kMenuSceneWinter, L"Winter");
+    AppendMenuW(sceneSubmenu_, MF_STRING, kMenuSceneAutumn, L"Autumn");
+    AppendMenuW(sceneSubmenu_, MF_STRING, kMenuSceneOcean,  L"Ocean");
+    AppendMenuW(trayMenu_, MF_POPUP | MF_STRING,
+                reinterpret_cast<UINT_PTR>(sceneSubmenu_), L"Scene");
+
+    critterSubmenu_ = CreatePopupMenu();
+    if (!critterSubmenu_) return false;
+    AppendMenuW(critterSubmenu_, MF_STRING, kMenuCritterNone,  L"None");
+    AppendMenuW(critterSubmenu_, MF_STRING, kMenuCritterSheep, L"Sheep");
+    AppendMenuW(critterSubmenu_, MF_STRING, kMenuCritterCat,   L"Cat");
+    AppendMenuW(critterSubmenu_, MF_STRING, kMenuCritterAll,   L"All");
+
+    petCountSubmenu_ = CreatePopupMenu();
+    if (!petCountSubmenu_) return false;
+    AppendMenuW(petCountSubmenu_, MF_STRING, kMenuPetCountRandom, L"Random");
+    for (int n : PET_COUNT_OPTIONS) {
+        AppendMenuW(petCountSubmenu_, MF_STRING,
+                    static_cast<UINT_PTR>(kMenuPetCount1 + (n - 1)),
+                    std::to_wstring(n).c_str());
+    }
+    AppendMenuW(critterSubmenu_, MF_SEPARATOR, 0, nullptr);
+    AppendMenuW(critterSubmenu_, MF_POPUP | MF_STRING,
+                reinterpret_cast<UINT_PTR>(petCountSubmenu_), L"Pet count");
+
+    AppendMenuW(trayMenu_, MF_POPUP | MF_STRING,
+                reinterpret_cast<UINT_PTR>(critterSubmenu_), L"Critter");
+
+    AppendMenuW(trayMenu_, MF_STRING, kMenuAutoStart, L"Start with Windows");
+    AppendMenuW(trayMenu_, MF_SEPARATOR, 0, nullptr);
+    AppendMenuW(trayMenu_, MF_STRING, kMenuQuit, L"Quit DesktopGrass");
+    UpdateSceneMenuCheck();
+    UpdateCritterMenuCheck();
+    UpdatePetCountMenuCheck();
+    UpdateAutoStartMenuCheck();
+
+    nid_ = {};
+    nid_.cbSize           = sizeof(nid_);
+    nid_.hWnd             = msgHwnd_;
+    nid_.uID              = kTrayIconId;
+    nid_.uFlags           = NIF_ICON | NIF_MESSAGE | NIF_TIP;
+    nid_.uCallbackMessage = kTrayMessage;
+
+    HICON icon = LoadIconW(hInst_, MAKEINTRESOURCEW(IDI_TRAYICON));
+    if (!icon) icon = LoadIconW(nullptr, IDI_APPLICATION);
+    nid_.hIcon = icon;
+    wcsncpy_s(nid_.szTip, L"Desktop Grass", _TRUNCATE);
+
+    BOOL ok = Shell_NotifyIconW(NIM_ADD, &nid_);
+    trayAdded_ = (ok == TRUE);
+    if (!trayAdded_) {
+        OutputDebugStringA("[DesktopGrass] Shell_NotifyIcon(NIM_ADD) failed\n");
+    }
+    return true; // non-fatal
+}
+
+void App::UpdateSceneMenuCheck() {
+    if (!sceneSubmenu_) return;
+    // Radio-style check: kMenuSceneGrass + Scene enum index.
+    const int activeId = kMenuSceneGrass + static_cast<int>(currentScene_);
+    CheckMenuRadioItem(sceneSubmenu_,
+                       kMenuSceneGrass, kMenuSceneOcean,
+                       activeId, MF_BYCOMMAND);
+}
+
+void App::SetScene(Scene s) {
+    if (s == currentScene_) {
+        UpdateSceneMenuCheck();
+        return;
+    }
+    currentScene_ = s;
+    for (auto& w : windows_) {
+        sim_set_scene(w->GetRenderer().GetSim(), s);
+    }
+    UpdateSceneMenuCheck();
+    SaveCurrentState();
+}
+
+void App::UpdateCritterMenuCheck() {
+    if (!critterSubmenu_) return;
+    const int activeId = kMenuCritterNone + static_cast<int>(currentCritter_);
+    CheckMenuRadioItem(critterSubmenu_,
+                       kMenuCritterNone, kMenuCritterAll,
+                       activeId, MF_BYCOMMAND);
+}
+
+void App::UpdatePetCountMenuCheck() {
+    if (!petCountSubmenu_) return;
+    const int activeId = currentCritterCount_ > 0
+        ? kMenuPetCount1 + (std::min(currentCritterCount_, PET_COUNT_MAX_PER_MONITOR) - 1)
+        : kMenuPetCountRandom;
+    CheckMenuRadioItem(petCountSubmenu_,
+                       kMenuPetCountRandom, kMenuPetCount6,
+                       activeId, MF_BYCOMMAND);
+}
+
+void App::UpdateAutoStartMenuCheck() {
+    if (!trayMenu_) return;
+    CheckMenuItem(trayMenu_, kMenuAutoStart,
+                  MF_BYCOMMAND | (autoStart_ ? MF_CHECKED : MF_UNCHECKED));
+}
+
+void App::SetAutoStart(bool enabled) {
+    if (enabled == autoStart_ && autostart::IsEnabled() == enabled) {
+        UpdateAutoStartMenuCheck();
+        return;
+    }
+
+    if (!autostart::SetEnabled(enabled)) {
+        OutputDebugStringA("[DesktopGrass] unable to update Start with Windows registry state\n");
+        UpdateAutoStartMenuCheck();
+        return;
+    }
+
+    autoStart_ = enabled;
+    UpdateAutoStartMenuCheck();
+    SaveCurrentState();
+}
+
+void App::SetCritter(CritterKind c) {
+    if (c == currentCritter_) {
+        UpdateCritterMenuCheck();
+        return;
+    }
+    currentCritter_ = c;
+    for (auto& w : windows_) {
+        sim_set_critter(w->GetRenderer().GetSim(), c);
+    }
+    UpdateCritterMenuCheck();
+    SaveCurrentState();
+}
+
+void App::SetCritterCount(int n) {
+    const int sanitized = n > 0 ? std::min(n, PET_COUNT_MAX_PER_MONITOR) : 0;
+    if (sanitized == currentCritterCount_) {
+        UpdatePetCountMenuCheck();
+        return;
+    }
+
+    currentCritterCount_ = sanitized;
+    for (auto& w : windows_) {
+        sim_set_critter_count(w->GetRenderer().GetSim(), currentCritterCount_);
+    }
+    UpdatePetCountMenuCheck();
+    SaveCurrentState();
+}
+
+void App::RemoveTrayIcon() {
+    if (trayAdded_) {
+        Shell_NotifyIconW(NIM_DELETE, &nid_);
+        trayAdded_ = false;
+    }
+}
+
+bool App::EnumerateMonitorsAndCreateWindows() {
+    DestroyAllGrassWindows();
+
+    MonitorEnumCtx ctx{ this, {}, {} };
+    EnumDisplayMonitors(nullptr, nullptr, MonitorEnumProc,
+                        reinterpret_cast<LPARAM>(&ctx));
+
+    if (ctx.bounds.empty()) return false;
+
+    for (size_t i = 0; i < ctx.bounds.size(); ++i) {
+        auto w = std::make_unique<GrassWindow>();
+        // Each monitor gets a deterministic seed derived from its physical
+        // origin (shared formula with Win2D) so blade patterns differ across
+        // monitors but stay stable across launches and line up with persisted
+        // cut records.
+        const uint64_t mseed = make_monitor_seed(ctx.bounds[i]);
+        if (w->Create(hInst_, ctx.bounds[i], ctx.dpis[i], mseed, config_.bladeDensity,
+                      config_.swaySpeed, config_.swayAmplitude)) {
+            ApplyPersistedStateToWindow(*w, ctx.bounds[i]);
+            w->Show();
+            windows_.push_back(std::move(w));
+        } else {
+            OutputDebugStringA("[DesktopGrass] GrassWindow::Create failed\n");
+        }
+    }
+    return !windows_.empty();
+}
+
+void App::DestroyAllGrassWindows() {
+    windows_.clear();
+}
+
+void App::OnDisplayChanged() {
+    SaveCurrentState();
+    EnumerateMonitorsAndCreateWindows();
+}
+
+void App::DispatchMouseEvents() {
+    // Drain the lock-free queue once. Each event is then routed to whichever
+    // GrassWindow's screen rect contains it.
+    RawMouseEvent raw[256];
+    while (true) {
+        std::size_t n = queue_.drain(raw, 256);
+        if (n == 0) break;
+
+        for (std::size_t i = 0; i < n; ++i) {
+            const RawMouseEvent& e = raw[i];
+            for (auto& w : windows_) {
+                const RECT& r = w->GetScreenBounds();
+                // Move events fire across the gust band; click events only
+                // when in the strip. The Sim's band-check (apply_move / click)
+                // re-filters in window-local coords. Here we route any event
+                // whose x is in the window's horizontal range — Move events
+                // need to update the prevCursor baseline even outside the
+                // band so the baseline stays accurate, and the spec already
+                // handles the band rejection.
+                if (e.screenX < r.left || e.screenX > r.right) continue;
+
+                // For move events we accept any y; for click events we only
+                // accept y inside the band.
+                if (e.type == EventType::Click) {
+                    if (e.screenY < r.top || e.screenY > r.bottom) continue;
+                }
+
+                // Convert to window-local DIPs.
+                const UINT dpi = w->GetRenderer().GetDpi();
+                const double scale = 96.0 / static_cast<double>(dpi);
+                InputEvent ie{};
+                ie.type = e.type;
+                ie.x    = (e.screenX - r.left) * scale;
+                ie.y    = (e.screenY - r.top)  * scale;
+                ie.time = e.timeSeconds;
+
+                // Apply directly to the sim. Note that this happens BEFORE
+                // sim_tick (which itself drains its events list), so we apply
+                // events through the per-tick path — collect into a per-window
+                // event vector instead.
+                // To keep things simple, push to the Sim immediately:
+                if (ie.type == EventType::Move) {
+                    sim_apply_move(w->GetRenderer().GetSim(), ie);
+                } else {
+                    sim_apply_click(w->GetRenderer().GetSim(), ie);
+                }
+                break; // each event belongs to at most one window
+            }
+        }
+        if (n < 256) break;
+    }
+}
+
+void App::RenderAllWindows(double dt) {
+    DispatchMouseEvents();
+    for (auto& w : windows_) {
+        w->RenderFrame(dt, nullptr, 0);
+    }
+}
+
+void App::ApplyPersistedStateToWindow(GrassWindow& window, const RECT& monitorBounds) {
+    Sim& sim = window.GetRenderer().GetSim();
+    sim_set_scene(sim, currentScene_);
+    sim_set_critter_count(sim, currentCritterCount_);
+    sim_set_critter(sim, currentCritter_);
+
+    if (!hasPersistedState_) return;
+
+    const int width = monitorBounds.right - monitorBounds.left;
+    const int height = monitorBounds.bottom - monitorBounds.top;
+    for (const persistence::MonitorState& monitor : persistedState_.monitors) {
+        if (monitor.width == width
+            && monitor.height == height
+            && monitor.left == monitorBounds.left
+            && monitor.top == monitorBounds.top) {
+            sim_apply_cuts(sim, monitor.cuts);
+            return;
+        }
+    }
+}
+
+persistence::AppState App::BuildAppState() {
+    persistence::AppState state;
+    state.version = 2;
+    state.scene = currentScene_;
+    state.critter = currentCritter_;
+    state.critterCountOverride = currentCritterCount_;
+    state.autoStart = autoStart_;
+
+    state.monitors.reserve(windows_.size());
+    for (const auto& w : windows_) {
+        const RECT& bounds = w->GetMonitorBounds();
+        persistence::MonitorState monitor;
+        monitor.width = bounds.right - bounds.left;
+        monitor.height = bounds.bottom - bounds.top;
+        monitor.left = bounds.left;
+        monitor.top = bounds.top;
+        const Sim& sim = w->GetRenderer().GetSim();
+        monitor.cuts = sim_get_cuts(sim);
+        state.monitors.push_back(std::move(monitor));
+    }
+
+    return state;
+}
+
+void App::SaveCurrentState() {
+    persistedState_ = BuildAppState();
+    hasPersistedState_ = true;
+    persistence::SaveAppState(persistedState_);
+    lastPersistenceSaveMs_ = GetTickCount64();
+}
+
+int App::Run() {
+    MSG msg{};
+    // Calm ambient content renders at the configured target fps (default 24
+    // via Config.h kTargetFpsDefault) to keep per-frame CPU low; motion is
+    // time-based (dt), so the rate only changes how often the same animation
+    // is sampled. The user can override this in config.json (targetFps).
+    const double kTargetFrameSec = 1.0 / static_cast<double>(config_.targetFps);
+
+    while (!quitRequested_) {
+        // Drain pending messages without blocking.
+        while (PeekMessageW(&msg, nullptr, 0, 0, PM_REMOVE)) {
+            if (msg.message == WM_QUIT) {
+                quitRequested_ = true;
+                break;
+            }
+            TranslateMessage(&msg);
+            DispatchMessageW(&msg);
+        }
+        if (quitRequested_) break;
+
+        // Compute dt.
+        LARGE_INTEGER now;
+        QueryPerformanceCounter(&now);
+        const double dt = static_cast<double>(now.QuadPart - qpcLast_.QuadPart) /
+                          static_cast<double>(qpcFreq_.QuadPart);
+        qpcLast_ = now;
+
+        RenderAllWindows(dt);
+
+        if (GetTickCount64() - lastPersistenceSaveMs_ >= 60000ull) {
+            SaveCurrentState();
+        }
+
+        // Pace to the target frame interval, accounting for the time already
+        // spent rendering/presenting this iteration so the cadence holds at
+        // the target fps regardless of how long Present blocked. The pacer
+        // uses a high-resolution waitable timer (Win 10 1803+) so the wait
+        // honours sub-15.6 ms remainders instead of getting clamped to the
+        // default system timer resolution. The wait returns early if input
+        // arrives, keeping the app responsive.
+        LARGE_INTEGER after;
+        QueryPerformanceCounter(&after);
+        const double elapsedSec = static_cast<double>(after.QuadPart - now.QuadPart) /
+                                  static_cast<double>(qpcFreq_.QuadPart);
+        const double remainingSec = kTargetFrameSec - elapsedSec;
+        pacer_.WaitUntilNextFrame(remainingSec);
+    }
+
+    SaveCurrentState();
+    return static_cast<int>(msg.wParam);
+}
+
+void App::RequestQuit() {
+    quitRequested_ = true;
+    PostQuitMessage(0);
+}
+
+LRESULT CALLBACK App::MessageWindowProc(HWND hwnd, UINT msg,
+                                        WPARAM wp, LPARAM lp)
+{
+    App* self = nullptr;
+    if (msg == WM_NCCREATE) {
+        auto* cs = reinterpret_cast<CREATESTRUCTW*>(lp);
+        self = reinterpret_cast<App*>(cs->lpCreateParams);
+        SetWindowLongPtrW(hwnd, GWLP_USERDATA, reinterpret_cast<LONG_PTR>(self));
+        if (self) self->msgHwnd_ = hwnd;
+    } else {
+        self = reinterpret_cast<App*>(GetWindowLongPtrW(hwnd, GWLP_USERDATA));
+    }
+    if (self) return self->HandleMessageWindowMessage(msg, wp, lp);
+    return DefWindowProcW(hwnd, msg, wp, lp);
+}
+
+LRESULT App::HandleMessageWindowMessage(UINT msg, WPARAM wp, LPARAM lp) {
+    switch (msg) {
+        case kTrayMessage:
+            if (LOWORD(lp) == WM_RBUTTONUP || LOWORD(lp) == WM_CONTEXTMENU) {
+                POINT pt;
+                GetCursorPos(&pt);
+                SetForegroundWindow(msgHwnd_);
+                TrackPopupMenu(trayMenu_,
+                               TPM_RIGHTBUTTON | TPM_BOTTOMALIGN,
+                               pt.x, pt.y, 0, msgHwnd_, nullptr);
+                PostMessageW(msgHwnd_, WM_NULL, 0, 0);
+            }
+            return 0;
+
+        case WM_COMMAND: {
+            const int id = LOWORD(wp);
+            if (id == kMenuPetCountRandom) {
+                SetCritterCount(0);
+                return 0;
+            }
+            if (id >= kMenuPetCount1 && id <= kMenuPetCount6) {
+                SetCritterCount(id - kMenuPetCount1 + 1);
+                return 0;
+            }
+            switch (id) {
+                case kMenuQuit:          RequestQuit();                       break;
+                case kMenuAutoStart:     SetAutoStart(!autoStart_);           break;
+                case kMenuSceneGrass:    SetScene(Scene::Grass);              break;
+                case kMenuSceneDesert:   SetScene(Scene::Desert);             break;
+                case kMenuSceneWinter:   SetScene(Scene::Winter);             break;
+                case kMenuSceneAutumn:   SetScene(Scene::Autumn);             break;
+    case kMenuSceneOcean:    SetScene(Scene::Ocean);              break;
+                case kMenuCritterNone:   SetCritter(CritterKind::None);       break;
+                case kMenuCritterSheep:  SetCritter(CritterKind::Sheep);      break;
+                case kMenuCritterCat:    SetCritter(CritterKind::Cat);        break;
+                case kMenuCritterAll:    SetCritter(CritterKind::Bunny);      break;
+            }
+            return 0;
+        }
+
+        case WM_DISPLAYCHANGE:
+            OnDisplayChanged();
+            return 0;
+
+        case WM_CLOSE:
+            // The smoke harness sends WM_CLOSE to the *grass* window, which
+            // PostQuitMessages from its WndProc. Also handle it here for
+            // robustness.
+            RequestQuit();
+            return 0;
+
+        default:
+            return DefWindowProcW(msgHwnd_, msg, wp, lp);
+    }
+}
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/App.h

@@ -0,0 +1,102 @@
+// App.h
+//
+// Application lifecycle. Owns the tray icon, the mouse hook, the per-monitor
+// GrassWindow list, and the message loop.
+
+#pragma once
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#include <shellapi.h>
+
+#include <memory>
+#include <vector>
+
+#include "GrassWindow.h"
+#include "MouseHook.h"
+#include "Pacing.h"
+#include "Persistence.h"
+#include "Config.h"
+
+namespace desktopgrass {
+
+class App {
+public:
+    static constexpr UINT  kTrayMessage     = WM_APP + 100;
+    static constexpr UINT  kTrayIconId      = 1;
+    static constexpr int   kMenuQuit          = 1001;
+    static constexpr int   kMenuSceneGrass    = 1010;
+    static constexpr int   kMenuSceneDesert   = 1011;
+    static constexpr int   kMenuSceneWinter   = 1012;
+    static constexpr int   kMenuSceneAutumn   = 1013;
+    static constexpr int   kMenuSceneOcean    = 1014;
+    static constexpr int   kMenuCritterNone     = 1020;
+    static constexpr int   kMenuCritterSheep    = 1021;
+    static constexpr int   kMenuCritterCat      = 1022;
+    static constexpr int   kMenuCritterAll      = 1023;
+    static constexpr int   kMenuPetCountRandom  = 1030;
+    static constexpr int   kMenuPetCount1       = 1031;
+    static constexpr int   kMenuPetCount6       = 1036;
+    static constexpr int   kMenuAutoStart       = 1040;
+
+    App() = default;
+    ~App();
+
+    bool Initialize(HINSTANCE hInst);
+    int  Run();
+    void RequestQuit();
+    void SetScene(Scene s);
+    Scene GetScene() const { return currentScene_; }
+    void SetCritter(CritterKind c);
+    CritterKind GetCritter() const { return currentCritter_; }
+    void SetCritterCount(int n);
+    int  GetCritterCount() const { return currentCritterCount_; }
+
+private:
+    bool CreateMessageWindow();
+    bool CreateTrayIcon();
+    void RemoveTrayIcon();
+    void DestroyMessageWindow();
+    bool EnumerateMonitorsAndCreateWindows();
+    void DestroyAllGrassWindows();
+    void OnDisplayChanged();
+    void DispatchMouseEvents();
+    void RenderAllWindows(double dt);
+    void ApplyPersistedStateToWindow(GrassWindow& window, const RECT& monitorBounds);
+    persistence::AppState BuildAppState();
+    void SaveCurrentState();
+    void SetAutoStart(bool enabled);
+    void UpdateSceneMenuCheck();
+    void UpdateCritterMenuCheck();
+    void UpdatePetCountMenuCheck();
+    void UpdateAutoStartMenuCheck();
+
+    static LRESULT CALLBACK MessageWindowProc(HWND hwnd, UINT msg,
+                                              WPARAM wp, LPARAM lp);
+    LRESULT HandleMessageWindowMessage(UINT msg, WPARAM wp, LPARAM lp);
+
+    HINSTANCE                                   hInst_   = nullptr;
+    HWND                                        msgHwnd_ = nullptr;
+    HMENU                                       trayMenu_ = nullptr;
+    HMENU                                       sceneSubmenu_ = nullptr;
+    HMENU                                       critterSubmenu_ = nullptr;
+    HMENU                                       petCountSubmenu_ = nullptr;
+    NOTIFYICONDATAW                             nid_{};
+    bool                                        trayAdded_ = false;
+    MouseEventQueue                             queue_{};
+    std::vector<std::unique_ptr<GrassWindow>>   windows_;
+    config::Config                              config_{};
+    Scene                                       currentScene_ = SCENE_DEFAULT;
+    CritterKind                                 currentCritter_ = CRITTER_DEFAULT;
+    int                                         currentCritterCount_ = 0;
+    bool                                        autoStart_ = false;
+    bool                                        hasPersistedState_ = false;
+    persistence::AppState                       persistedState_{};
+    ULONGLONG                                   lastPersistenceSaveMs_ = 0;
+    LARGE_INTEGER                               qpcFreq_{};
+    LARGE_INTEGER                               qpcLast_{};
+    FramePacer                                  pacer_{};
+    bool                                        quitRequested_ = false;
+};
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/AutoStart.cpp

@@ -0,0 +1,107 @@
+#include "AutoStart.h"
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+#include <vector>
+
+#pragma comment(lib, "Advapi32.lib")
+
+namespace autostart {
+namespace {
+
+constexpr const wchar_t* kDefaultRunSubKey = L"Software\\Microsoft\\Windows\\CurrentVersion\\Run";
+std::wstring g_registryKeyOverride;
+
+std::wstring GetRegistrySubKey() {
+    return g_registryKeyOverride.empty() ? std::wstring(kDefaultRunSubKey) : g_registryKeyOverride;
+}
+
+} // namespace
+
+std::wstring GetRegistryValueName() {
+    return L"DesktopGrass.Native";
+}
+
+std::wstring GetCurrentExePath() {
+    std::vector<wchar_t> buffer(MAX_PATH);
+    while (true) {
+        const DWORD length = GetModuleFileNameW(nullptr, buffer.data(), static_cast<DWORD>(buffer.size()));
+        if (length == 0) {
+            return L"";
+        }
+        if (length < buffer.size()) {
+            return std::wstring(buffer.data(), length);
+        }
+        buffer.resize(buffer.size() * 2);
+    }
+}
+
+bool IsEnabled() {
+    HKEY key = nullptr;
+    const std::wstring subKey = GetRegistrySubKey();
+    const LSTATUS openStatus = RegOpenKeyExW(
+        HKEY_CURRENT_USER, subKey.c_str(), 0, KEY_QUERY_VALUE, &key);
+    if (openStatus != ERROR_SUCCESS) {
+        return false;
+    }
+
+    DWORD type = 0;
+    const std::wstring valueName = GetRegistryValueName();
+    const LSTATUS queryStatus = RegQueryValueExW(
+        key, valueName.c_str(), nullptr, &type, nullptr, nullptr);
+    RegCloseKey(key);
+
+    return queryStatus == ERROR_SUCCESS && (type == REG_SZ || type == REG_EXPAND_SZ);
+}
+
+bool SetEnabled(bool on) {
+    const std::wstring subKey = GetRegistrySubKey();
+    const std::wstring valueName = GetRegistryValueName();
+
+    if (on) {
+        const std::wstring path = GetCurrentExePath();
+        if (path.empty()) {
+            return false;
+        }
+
+        HKEY key = nullptr;
+        const LSTATUS createStatus = RegCreateKeyExW(
+            HKEY_CURRENT_USER, subKey.c_str(), 0, nullptr, REG_OPTION_NON_VOLATILE,
+            KEY_SET_VALUE, nullptr, &key, nullptr);
+        if (createStatus != ERROR_SUCCESS) {
+            return false;
+        }
+
+        const DWORD byteCount = static_cast<DWORD>((path.size() + 1) * sizeof(wchar_t));
+        const LSTATUS setStatus = RegSetValueExW(
+            key, valueName.c_str(), 0, REG_SZ,
+            reinterpret_cast<const BYTE*>(path.c_str()), byteCount);
+        RegCloseKey(key);
+        return setStatus == ERROR_SUCCESS;
+    }
+
+    HKEY key = nullptr;
+    const LSTATUS openStatus = RegOpenKeyExW(
+        HKEY_CURRENT_USER, subKey.c_str(), 0, KEY_SET_VALUE, &key);
+    if (openStatus == ERROR_FILE_NOT_FOUND) {
+        return true;
+    }
+    if (openStatus != ERROR_SUCCESS) {
+        return false;
+    }
+
+    const LSTATUS deleteStatus = RegDeleteValueW(key, valueName.c_str());
+    RegCloseKey(key);
+    return deleteStatus == ERROR_SUCCESS || deleteStatus == ERROR_FILE_NOT_FOUND;
+}
+
+bool ReconcileWithState(bool autoStart) {
+    return IsEnabled() == autoStart || SetEnabled(autoStart);
+}
+
+void SetRegistryKeyOverride(const std::wstring& subkey) {
+    g_registryKeyOverride = subkey;
+}
+
+} // namespace autostart

src/modules/DesktopGrass/DesktopGrass.Native/src/AutoStart.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <string>
+
+namespace autostart {
+
+bool IsEnabled();
+bool SetEnabled(bool on);
+std::wstring GetRegistryValueName();
+std::wstring GetCurrentExePath();
+bool ReconcileWithState(bool autoStart);
+void SetRegistryKeyOverride(const std::wstring& subkey);
+
+} // namespace autostart

src/modules/DesktopGrass/DesktopGrass.Native/src/Benchmark.cpp

@@ -0,0 +1,328 @@
+// Benchmark.cpp
+//
+// See Benchmark.h. Minimal, side-effect-free measurement runner.
+
+#include "Benchmark.h"
+
+#include "GrassWindow.h"
+#include "Pacing.h"
+#include "Sim.h"
+
+#include <shellscalingapi.h>
+
+#include <algorithm>
+#include <cstdio>
+#include <cwchar>
+#include <cstring>
+#include <string>
+
+#pragma comment(lib, "Shcore.lib")
+#pragma comment(lib, "User32.lib")
+
+namespace desktopgrass::benchmark {
+
+namespace {
+
+// Same seed used by the production App so blade layouts are bit-identical
+// across production runs and benchmark runs at the same monitor origin.
+constexpr uint64_t kBenchmarkDefaultSeed = 0xD3C7C0F30070D511ull;
+
+bool ParseInt(const wchar_t* s, int& out) {
+    if (!s || !*s) return false;
+    wchar_t* end = nullptr;
+    long v = std::wcstol(s, &end, 10);
+    if (end == s || *end != L'\0') return false;
+    out = static_cast<int>(v);
+    return true;
+}
+
+bool ParseU64(const wchar_t* s, uint64_t& out) {
+    if (!s || !*s) return false;
+    wchar_t* end = nullptr;
+    int base = 10;
+    if (s[0] == L'0' && (s[1] == L'x' || s[1] == L'X')) {
+        base = 16;
+        s += 2;
+    }
+    unsigned long long v = std::wcstoull(s, &end, base);
+    if (end == s || *end != L'\0') return false;
+    out = static_cast<uint64_t>(v);
+    return true;
+}
+
+bool ParseScene(const wchar_t* s, Scene& out) {
+    int n = -1;
+    if (ParseInt(s, n) && n >= 0 && n < SCENE_COUNT) {
+        out = static_cast<Scene>(n);
+        return true;
+    }
+    if (_wcsicmp(s, L"grass")  == 0) { out = Scene::Grass;  return true; }
+    if (_wcsicmp(s, L"desert") == 0) { out = Scene::Desert; return true; }
+    if (_wcsicmp(s, L"winter") == 0) { out = Scene::Winter; return true; }
+    if (_wcsicmp(s, L"autumn") == 0) { out = Scene::Autumn; return true; }
+    if (_wcsicmp(s, L"ocean")  == 0) { out = Scene::Ocean;  return true; }
+    return false;
+}
+
+bool ParseCritter(const wchar_t* s, CritterKind& out) {
+    int n = -1;
+    if (ParseInt(s, n) && n >= 0 && n < CRITTER_COUNT) {
+        out = static_cast<CritterKind>(n);
+        return true;
+    }
+    if (_wcsicmp(s, L"none")  == 0) { out = CritterKind::None;  return true; }
+    if (_wcsicmp(s, L"sheep") == 0) { out = CritterKind::Sheep; return true; }
+    if (_wcsicmp(s, L"cat")   == 0) { out = CritterKind::Cat;   return true; }
+    if (_wcsicmp(s, L"bunny") == 0) { out = CritterKind::Bunny; return true; }
+    if (_wcsicmp(s, L"all")   == 0) { out = CritterKind::Bunny; return true; }
+    return false;
+}
+
+// Split an arg of the form `--key=value` into (key, value). If the arg is
+// `--key` with the value in the next arg, value is set to the next arg and
+// `consumedExtra` is true.
+struct KV {
+    std::wstring key;
+    const wchar_t* value = nullptr;
+    bool consumedExtra = false;
+};
+
+KV SplitArg(const wchar_t* arg, const wchar_t* nextArg) {
+    KV kv;
+    if (!arg || arg[0] != L'-' || arg[1] != L'-') return kv;
+    const wchar_t* eq = std::wcschr(arg, L'=');
+    if (eq) {
+        kv.key.assign(arg + 2, eq);
+        kv.value = eq + 1;
+    } else {
+        kv.key.assign(arg + 2);
+        if (nextArg && nextArg[0] != L'-') {
+            kv.value = nextArg;
+            kv.consumedExtra = true;
+        }
+    }
+    return kv;
+}
+
+// Get the primary monitor's work area + effective DPI.
+bool GetPrimaryMonitorInfo(RECT& workOut, UINT& dpiOut) {
+    POINT origin{ 0, 0 };
+    HMONITOR mon = MonitorFromPoint(origin, MONITOR_DEFAULTTOPRIMARY);
+    if (!mon) return false;
+    MONITORINFO mi{};
+    mi.cbSize = sizeof(mi);
+    if (!GetMonitorInfoW(mon, &mi)) return false;
+    workOut = mi.rcWork;
+    UINT xDpi = 96, yDpi = 96;
+    if (FAILED(GetDpiForMonitor(mon, MDT_EFFECTIVE_DPI, &xDpi, &yDpi))) {
+        xDpi = 96;
+    }
+    dpiOut = xDpi;
+    return true;
+}
+
+} // anonymous
+
+bool ParseOptions(int argc, wchar_t** argv, Options& out) {
+    // argv[0] is the executable; start at 1.
+    for (int i = 1; i < argc; ++i) {
+        const wchar_t* arg = argv[i];
+        if (!arg || arg[0] != L'-' || arg[1] != L'-') continue;
+        // --benchmark is handled by the caller (it's the mode switch) — skip.
+        if (_wcsicmp(arg, L"--benchmark") == 0) continue;
+
+        const wchar_t* nextArg = (i + 1 < argc) ? argv[i + 1] : nullptr;
+        KV kv = SplitArg(arg, nextArg);
+        if (kv.key.empty()) continue;
+        if (kv.consumedExtra) ++i;
+        if (!kv.value) continue;
+
+        if (_wcsicmp(kv.key.c_str(), L"scene") == 0) {
+            if (!ParseScene(kv.value, out.scene)) return false;
+        } else if (_wcsicmp(kv.key.c_str(), L"critter") == 0) {
+            if (!ParseCritter(kv.value, out.critter)) return false;
+        } else if (_wcsicmp(kv.key.c_str(), L"critter-count") == 0 ||
+                   _wcsicmp(kv.key.c_str(), L"crittercount") == 0) {
+            if (!ParseInt(kv.value, out.critterCount)) return false;
+        } else if (_wcsicmp(kv.key.c_str(), L"seed") == 0) {
+            if (!ParseU64(kv.value, out.seed)) return false;
+        } else if (_wcsicmp(kv.key.c_str(), L"duration") == 0) {
+            if (!ParseInt(kv.value, out.durationSec)) return false;
+            if (out.durationSec < 1) out.durationSec = 1;
+        } else if (_wcsicmp(kv.key.c_str(), L"width") == 0) {
+            if (!ParseInt(kv.value, out.widthPx)) return false;
+        } else if (_wcsicmp(kv.key.c_str(), L"height") == 0) {
+            if (!ParseInt(kv.value, out.heightPx)) return false;
+        } else if (_wcsicmp(kv.key.c_str(), L"fps") == 0 ||
+                   _wcsicmp(kv.key.c_str(), L"target-fps") == 0) {
+            if (!ParseInt(kv.value, out.targetFps)) return false;
+            if (out.targetFps < 1) out.targetFps = 1;
+        } else if (_wcsicmp(kv.key.c_str(), L"out") == 0 ||
+                   _wcsicmp(kv.key.c_str(), L"csv") == 0) {
+            out.outCsvPath.assign(kv.value);
+        } else if (_wcsicmp(kv.key.c_str(), L"hidden") == 0 ||
+                   _wcsicmp(kv.key.c_str(), L"hide") == 0) {
+            // Treat presence of the flag as `true`. Accept explicit value too.
+            int v = 1;
+            ParseInt(kv.value, v);
+            out.hideWindow = (v != 0);
+        }
+        // Unknown flags are silently ignored — older drivers stay compatible
+        // with newer binaries that add flags.
+    }
+    return true;
+}
+
+int Run(HINSTANCE hInst, const Options& opts) {
+    RECT primaryWork{};
+    UINT primaryDpi = 96;
+    if (!GetPrimaryMonitorInfo(primaryWork, primaryDpi)) {
+        std::fwprintf(stderr, L"[benchmark] failed to query primary monitor\n");
+        return 1;
+    }
+
+    const int primaryW = primaryWork.right - primaryWork.left;
+    const int primaryH = primaryWork.bottom - primaryWork.top;
+    const int targetW  = opts.widthPx  > 0 ? opts.widthPx  : primaryW;
+    const int defaultStripPx = static_cast<int>(
+        ((STRIP_HEIGHT + HEADROOM) * primaryDpi / 96.0) + 0.5);
+    const int targetH  = opts.heightPx > 0 ? opts.heightPx : defaultStripPx;
+
+    // GrassWindow.Create takes a "monitor work area" rect; it derives window
+    // origin/size from it. To honour width/height overrides while keeping the
+    // window pinned to the bottom-left of the primary work area, fabricate a
+    // monitorBounds rect of the requested width whose bottom matches the
+    // primary work-area bottom. Height is forced via the strip-DIP formula
+    // inside GrassWindow.Create; if the caller asked for a non-default
+    // heightPx we ignore that for the actual HWND (the renderer always uses
+    // STRIP_HEIGHT + HEADROOM in DIPs), but we still log targetH in the CSV
+    // header for traceability.
+    RECT monitorBounds = primaryWork;
+    monitorBounds.right = monitorBounds.left + targetW;
+    (void)targetH; // logged in CSV header below; HWND height is fixed by spec
+
+    if (!GrassWindow::RegisterWindowClass(hInst)) {
+        std::fwprintf(stderr, L"[benchmark] RegisterWindowClass failed\n");
+        return 1;
+    }
+
+    const uint64_t seed = opts.seed != 0 ? opts.seed : kBenchmarkDefaultSeed;
+
+    GrassWindow window;
+    if (!window.Create(hInst, monitorBounds, primaryDpi, seed,
+                       /*density=*/1.0, /*swaySpeed=*/1.0, /*swayAmplitude=*/1.0)) {
+        std::fwprintf(stderr, L"[benchmark] GrassWindow::Create failed\n");
+        return 1;
+    }
+
+    if (!opts.hideWindow) {
+        window.Show();
+    }
+
+    Sim& sim = window.GetRenderer().GetSim();
+    sim_set_scene(sim, opts.scene);
+    sim_set_critter_count(sim, opts.critterCount);
+    sim_set_critter(sim, opts.critter);
+
+    // Optional per-frame CSV.
+    FILE* csv = nullptr;
+    if (!opts.outCsvPath.empty()) {
+        if (_wfopen_s(&csv, opts.outCsvPath.c_str(), L"w, ccs=UTF-8") != 0 || !csv) {
+            std::fwprintf(stderr, L"[benchmark] failed to open %ls for write\n",
+                          opts.outCsvPath.c_str());
+            return 1;
+        }
+        std::fwprintf(csv,
+            L"# scene=%d critter=%d critter_count=%d seed=0x%016llX "
+            L"duration_s=%d target_fps=%d width_px=%d height_px=%d dpi=%u "
+            L"primary_work=%dx%d\n",
+            static_cast<int>(opts.scene),
+            static_cast<int>(opts.critter),
+            opts.critterCount,
+            static_cast<unsigned long long>(seed),
+            opts.durationSec, opts.targetFps, targetW, targetH,
+            primaryDpi, primaryW, primaryH);
+        std::fwprintf(csv, L"frame_index,t_seconds,dt_ms,render_ms\n");
+    }
+
+    LARGE_INTEGER qpcFreq{};
+    QueryPerformanceFrequency(&qpcFreq);
+    const double freq = static_cast<double>(qpcFreq.QuadPart);
+
+    LARGE_INTEGER tStart{};
+    QueryPerformanceCounter(&tStart);
+    LARGE_INTEGER tPrev = tStart;
+
+    const double durationS = static_cast<double>(opts.durationSec);
+    const double targetFrameSec = 1.0 / static_cast<double>(opts.targetFps);
+
+    long long frameIndex = 0;
+    bool userClosed = false;
+    FramePacer pacer;
+    int  exitCode = 0;
+
+    MSG msg{};
+    while (true) {
+        while (PeekMessageW(&msg, nullptr, 0, 0, PM_REMOVE)) {
+            if (msg.message == WM_QUIT) {
+                userClosed = true;
+                break;
+            }
+            TranslateMessage(&msg);
+            DispatchMessageW(&msg);
+        }
+        if (userClosed) break;
+
+        LARGE_INTEGER tNow{};
+        QueryPerformanceCounter(&tNow);
+        const double elapsedSinceStart =
+            static_cast<double>(tNow.QuadPart - tStart.QuadPart) / freq;
+        if (elapsedSinceStart >= durationS) break;
+
+        const double dt = static_cast<double>(tNow.QuadPart - tPrev.QuadPart) / freq;
+        tPrev = tNow;
+
+        LARGE_INTEGER tRenderStart{};
+        QueryPerformanceCounter(&tRenderStart);
+        window.RenderFrame(dt, nullptr, 0);
+        LARGE_INTEGER tRenderEnd{};
+        QueryPerformanceCounter(&tRenderEnd);
+
+        const double renderMs =
+            static_cast<double>(tRenderEnd.QuadPart - tRenderStart.QuadPart) * 1000.0 / freq;
+
+        if (csv) {
+            std::fwprintf(csv, L"%lld,%.6f,%.4f,%.4f\n",
+                          frameIndex, elapsedSinceStart, dt * 1000.0, renderMs);
+        }
+        ++frameIndex;
+
+        // Pace to target fps via the shared high-resolution waitable timer.
+        LARGE_INTEGER tAfter{};
+        QueryPerformanceCounter(&tAfter);
+        const double spentSec =
+            static_cast<double>(tAfter.QuadPart - tNow.QuadPart) / freq;
+        const double remaining = targetFrameSec - spentSec;
+        pacer.WaitUntilNextFrame(remaining);
+    }
+
+    if (csv) std::fclose(csv);
+
+    if (userClosed) exitCode = 2;
+
+    // Print one-line summary so the harness can capture even without parsing
+    // the CSV — written to stdout so it shows up in run logs.
+    LARGE_INTEGER tEnd{};
+    QueryPerformanceCounter(&tEnd);
+    const double totalSec =
+        static_cast<double>(tEnd.QuadPart - tStart.QuadPart) / freq;
+    const double effectiveFps = frameIndex > 0 && totalSec > 0
+        ? static_cast<double>(frameIndex) / totalSec
+        : 0.0;
+    std::wprintf(L"[benchmark] scene=%d frames=%lld duration_s=%.3f fps=%.2f exit=%d\n",
+                 static_cast<int>(opts.scene), frameIndex, totalSec, effectiveFps,
+                 exitCode);
+    return exitCode;
+}
+
+} // namespace desktopgrass::benchmark

src/modules/DesktopGrass/DesktopGrass.Native/src/Benchmark.h

@@ -0,0 +1,55 @@
+// Benchmark.h
+//
+// Optional benchmark entry point used by tools/benchmark/ to measure renderer
+// cost in a deterministic, headless-ish run. NOT compiled into the production
+// path — main.cpp only invokes this when `--benchmark` appears on the command
+// line. Production code is untouched.
+//
+// The benchmark:
+//   * Creates ONE GrassWindow on the primary monitor's bottom strip (visible,
+//     same DComp/Direct2D path users see).
+//   * Skips the tray icon, MouseHook, persistence, and multi-monitor enum.
+//   * Forces a fixed scene + critter + seed so blade/entity content is
+//     reproducible across runs.
+//   * Renders for the requested duration, capturing per-frame CPU-side
+//     timings to a CSV.
+//   * Exits cleanly so an external driver can collect counters and move on.
+
+#pragma once
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+#include <cstdint>
+#include <string>
+
+#include "Constants.h"
+
+namespace desktopgrass::benchmark {
+
+struct Options {
+    Scene        scene        = Scene::Grass;
+    CritterKind  critter      = CRITTER_DEFAULT;
+    int          critterCount = 0;      // 0 = scene default (random species count)
+    uint64_t     seed         = 0;      // 0 = use the production app seed
+    int          durationSec  = 60;
+    int          widthPx      = 0;      // 0 = primary-monitor work-area width
+    int          heightPx     = 0;      // 0 = STRIP_HEIGHT + HEADROOM at primary DPI
+    int          targetFps    = 24;     // matches Config.h kTargetFpsDefault; overridable
+    std::wstring outCsvPath;            // empty = no per-frame log written
+    bool         hideWindow   = false;  // SW_HIDE instead of SW_SHOWNOACTIVATE
+};
+
+// Parse `--benchmark`-mode args. argv[0] should be the executable; subsequent
+// entries are recognized in the form `--key=value` or `--key value`. Unknown
+// flags are silently ignored so future production flags don't break older
+// harness invocations. Returns false if a required value is malformed.
+bool ParseOptions(int argc, wchar_t** argv, Options& out);
+
+// Run a single benchmark using `opts`. Returns the process exit code:
+//   0  -> ran for the full duration
+//   1  -> setup failure (window/renderer init, output CSV open)
+//   2  -> early exit (user closed the window before duration elapsed)
+int Run(HINSTANCE hInst, const Options& opts);
+
+} // namespace desktopgrass::benchmark

src/modules/DesktopGrass/DesktopGrass.Native/src/Config.cpp

@@ -0,0 +1,150 @@
+// Config.cpp
+
+#include "Config.h"
+
+#include "Json.h"
+
+#include <Windows.h>
+
+#include <algorithm>
+#include <cmath>
+#include <cstdlib>
+#include <filesystem>
+#include <fstream>
+#include <sstream>
+#include <string>
+
+namespace desktopgrass::config {
+namespace {
+
+// Annotated default config written verbatim on first run. JSONC comments are
+// tolerated by the loader, so users can keep these notes while editing.
+constexpr char kDefaultConfigTemplate[] =
+    "{\n"
+    "  // DesktopGrass settings. Edit and restart the app to apply.\n"
+    "  // This file is created once and never overwritten, so your edits stick.\n"
+    "  \"version\": 1,\n"
+    "\n"
+    "  // Animation frame rate. Lower = less CPU, choppier motion. Range 5-144.\n"
+    "  \"targetFps\": 24,\n"
+    "\n"
+    "  // Grass blade density. Lower = fewer blades (less CPU). Range 0.2-5.0.\n"
+    "  // Default 2.53125.\n"
+    "  \"bladeDensity\": 2.53125,\n"
+    "\n"
+    "  // Grass sway speed multiplier. 1.0 = default, 0.0 = still, higher = faster.\n"
+    "  // Range 0.0-3.0.\n"
+    "  \"swaySpeed\": 1.0,\n"
+    "\n"
+    "  // Grass sway amplitude multiplier (how far blades lean). 1.0 = default,\n"
+    "  // 0.0 = upright. Range 0.0-3.0.\n"
+    "  \"swayAmplitude\": 1.0\n"
+    "}\n";
+
+std::wstring DefaultConfigFilePath() {
+    wchar_t* localAppData = nullptr;
+    std::size_t length = 0;
+    _wdupenv_s(&localAppData, &length, L"LOCALAPPDATA");
+
+    std::filesystem::path path = localAppData && length > 0
+        ? std::filesystem::path(localAppData)
+        : std::filesystem::current_path();
+
+    if (localAppData) {
+        std::free(localAppData);
+    }
+
+    path /= L"DesktopGrass";
+    path /= L"config.json";
+    return path.wstring();
+}
+
+// Writes the annotated default config, but only if no file exists yet. Uses
+// CREATE_NEW so a concurrent writer or an existing user file is never clobbered.
+void TryWriteDefaultConfig(const std::filesystem::path& path) {
+    const std::filesystem::path directory = path.parent_path();
+    if (!directory.empty()) {
+        std::error_code ec;
+        std::filesystem::create_directories(directory, ec);
+    }
+
+    HANDLE handle = CreateFileW(path.c_str(), GENERIC_WRITE, 0, nullptr,
+                                CREATE_NEW, FILE_ATTRIBUTE_NORMAL, nullptr);
+    if (handle == INVALID_HANDLE_VALUE) {
+        return; // Already exists (or unwritable): leave it untouched.
+    }
+
+    DWORD written = 0;
+    WriteFile(handle, kDefaultConfigTemplate,
+              static_cast<DWORD>(sizeof(kDefaultConfigTemplate) - 1), &written, nullptr);
+    CloseHandle(handle);
+}
+
+template <typename T>
+T Clamp(T value, T lo, T hi) {
+    return value < lo ? lo : (value > hi ? hi : value);
+}
+
+// Clamps a config double, treating non-finite values (NaN/inf from malformed
+// input) as the supplied default so they can never poison the sim.
+double ClampFinite(double value, double lo, double hi, double fallback) {
+    if (!std::isfinite(value)) return fallback;
+    return Clamp(value, lo, hi);
+}
+
+Config ApplyAndClamp(const json::Value& root) {
+    Config cfg;
+    cfg.version = json::ReadInt(root, "version").value_or(kConfigVersion);
+
+    const int fps = json::ReadInt(root, "targetFps").value_or(kTargetFpsDefault);
+    cfg.targetFps = Clamp(fps, kTargetFpsMin, kTargetFpsMax);
+
+    const double density = json::ReadDouble(root, "bladeDensity").value_or(kBladeDensityDefault);
+    cfg.bladeDensity = ClampFinite(density, kBladeDensityMin, kBladeDensityMax, kBladeDensityDefault);
+
+    const double swaySpeed = json::ReadDouble(root, "swaySpeed").value_or(kSwaySpeedDefault);
+    cfg.swaySpeed = ClampFinite(swaySpeed, kSwaySpeedMin, kSwaySpeedMax, kSwaySpeedDefault);
+
+    const double swayAmp = json::ReadDouble(root, "swayAmplitude").value_or(kSwayAmplitudeDefault);
+    cfg.swayAmplitude = ClampFinite(swayAmp, kSwayAmplitudeMin, kSwayAmplitudeMax, kSwayAmplitudeDefault);
+
+    return cfg;
+}
+
+} // namespace
+
+std::wstring GetConfigFilePath() {
+    return DefaultConfigFilePath();
+}
+
+Config LoadConfig(const std::wstring& pathStr) {
+    const std::filesystem::path path(pathStr);
+
+    if (!std::filesystem::exists(path)) {
+        TryWriteDefaultConfig(path);
+        return Config{}; // Defaults match the template we just wrote.
+    }
+
+    std::ifstream file(path, std::ios::binary);
+    if (!file) {
+        return Config{};
+    }
+
+    std::ostringstream buffer;
+    buffer << file.rdbuf();
+    const std::string text = buffer.str();
+
+    json::Value root;
+    if (!json::Parse(text, root) || root.type != json::Value::Type::Object) {
+        OutputDebugStringA("DesktopGrass config: malformed config.json; using defaults.\n");
+        return Config{}; // Preserve the user's file; just fall back to defaults.
+    }
+
+    return ApplyAndClamp(root);
+}
+
+Config LoadConfig() {
+    return LoadConfig(GetConfigFilePath());
+}
+
+} // namespace desktopgrass::config

src/modules/DesktopGrass/DesktopGrass.Native/src/Config.h

@@ -0,0 +1,50 @@
+// Config.h
+//
+// User-editable settings loaded from config.json. Distinct from state.json
+// (which the app owns and rewrites): config.json is written once with annotated
+// defaults if missing, and thereafter only ever read — never overwritten — so a
+// user's hand edits are preserved. Loaded once at startup.
+
+#pragma once
+
+#include "Constants.h"
+
+#include <string>
+
+namespace desktopgrass::config {
+
+// Valid ranges and defaults for the exposed knobs. Defaults reproduce the app's
+// historical behavior exactly.
+constexpr int    kConfigVersion        = 1;
+constexpr int    kTargetFpsDefault      = 24;
+constexpr int    kTargetFpsMin          = 5;
+constexpr int    kTargetFpsMax          = 144;
+constexpr double kBladeDensityDefault   = DEFAULT_DENSITY; // 2.53125
+constexpr double kBladeDensityMin       = 0.2;
+constexpr double kBladeDensityMax       = 5.0;
+constexpr double kSwaySpeedDefault      = 1.0;
+constexpr double kSwaySpeedMin          = 0.0;
+constexpr double kSwaySpeedMax          = 3.0;
+constexpr double kSwayAmplitudeDefault  = 1.0;
+constexpr double kSwayAmplitudeMin      = 0.0;
+constexpr double kSwayAmplitudeMax      = 3.0;
+
+struct Config {
+    int    version       = kConfigVersion;
+    int    targetFps     = kTargetFpsDefault;
+    double bladeDensity  = kBladeDensityDefault;
+    double swaySpeed     = kSwaySpeedDefault;
+    double swayAmplitude = kSwayAmplitudeDefault;
+};
+
+// Loads config.json from the default location, creating an annotated default
+// file if it is missing. Returns clamped, validated values; on any error falls
+// back to defaults without overwriting an existing file. Always succeeds.
+Config LoadConfig();
+
+// Path overload for tests: reads/creates the config at the given path.
+Config LoadConfig(const std::wstring& path);
+
+std::wstring GetConfigFilePath();
+
+} // namespace desktopgrass::config

src/modules/DesktopGrass/DesktopGrass.Native/src/Constants.h

@@ -0,0 +1,988 @@
+// Constants.h
+//
+// Single source of truth for all simulation constants.
+// Mirrors docs/architecture.md §11. If a constant changes here it MUST change
+// in the spec first.
+
+#pragma once
+
+#include <algorithm>
+#include <cmath>
+#include <cstdint>
+
+namespace desktopgrass {
+
+// Geometry --------------------------------------------------------------------
+constexpr double STRIP_HEIGHT          = 80.0;
+constexpr double HEADROOM              = 30.0;
+
+// Procedural generation -------------------------------------------------------
+constexpr double DEFAULT_DENSITY        = 2.53125;
+constexpr double BLADE_SPACING_MIN     = 4.0;
+constexpr double BLADE_SPACING_MAX     = 8.0;
+constexpr double BLADE_HEIGHT_MIN      = 6.0;
+constexpr double BLADE_HEIGHT_MAX      = 30.0;
+constexpr double BLADE_THICKNESS_MIN   = 1.0;
+constexpr double BLADE_THICKNESS_MAX   = 2.5;
+// Render-only stroke-width bonus added to each blade so grass reads thicker
+// on screen without perturbing the generation PRNG / blade snapshots.
+constexpr double BLADE_THICKNESS_RENDER_BONUS = 1.5;
+constexpr double STIFFNESS_MIN         = 0.6;
+constexpr double STIFFNESS_MAX         = 1.0;
+constexpr int    PALETTE_SIZE          = 6;
+
+// Sway / gust physics ---------------------------------------------------------
+// π / 3 → 6-second sway period.
+constexpr double BASE_SWAY_SPEED       = 1.0471975511965976;
+constexpr double BASE_AMPLITUDE        = 3.3;
+constexpr double DECAY_RATE            = 2.5;
+constexpr double GUST_TO_LEAN_FACTOR   = 0.75;
+constexpr double MAX_CURSOR_SPEED      = 4000.0;
+constexpr double IMPULSE_SCALE         = 0.003;
+constexpr double GUST_RADIUS           = 150.0;
+constexpr double CURSOR_REINIT_GAP_SEC = 0.25;
+
+// Cut ------------------------------------------------------------------------
+constexpr double CUT_RADIUS            = 15.0;
+constexpr double CUT_DURATION_SEC      = 0.2;
+constexpr double CUT_STUMP_THRESHOLD   = 0.05;
+constexpr double STUMP_HEIGHT          = 2.0;
+constexpr double MUSHROOM_STUMP_HEIGHT = 4.0;  // §7 — sits a touch above the grass stub line
+constexpr double CTRL_OFFSET_FACTOR    = 0.6;
+// fraction of blade length that the tip may horizontally displace; clamps gust impulses so the blade never folds completely flat.
+constexpr double MAX_LEAN_FRACTION     = 0.95;
+
+// Cut residual height (stubble). A freshly mowed blade does not collapse to a
+// flat stump; it settles at a small per-blade normalized height in
+// [CUT_FLOOR_MIN, CUT_FLOOR_MAX] so the cut line reads with gentle, natural
+// variation instead of a perfectly even edge. Both bounds stay above
+// CUT_STUMP_THRESHOLD so stubble renders as a short blade (still swaying), not
+// a degenerate stump. Sampled from an independent stream salted with
+// CUT_FLOOR_PRNG_SALT so it does NOT perturb the main generation sequence.
+constexpr double   CUT_FLOOR_MIN       = 0.06;
+constexpr double   CUT_FLOOR_MAX       = 0.16;
+constexpr uint64_t CUT_FLOOR_PRNG_SALT = 0xC07F100DC07F100Dull;
+
+// Regrowth -------------------------------------------------------------------
+// After a blade's cut animation finishes, it waits `regrowDelay` seconds (a
+// per-blade jittered value in [MIN, MAX]) and then grows back from cutHeight=0
+// to cutHeight=1 linearly over `regrowDuration` seconds (also per-blade
+// jittered). The jitter is sampled from a second xorshift64 stream seeded with
+// `seed XOR REGROW_PRNG_SALT` so it does NOT perturb blade positions/heights
+// drawn from the main stream — conformance with seed 0x6B6173746F is preserved.
+constexpr double REGROW_DELAY_MIN      = 30.0;
+constexpr double REGROW_DELAY_MAX      = 90.0;
+constexpr double REGROW_DURATION_MIN   = 2.0;
+constexpr double REGROW_DURATION_MAX   = 4.0;
+constexpr uint64_t REGROW_PRNG_SALT    = 0xDEADBEEFCAFEBABEull;
+
+// Flowers (§4, §5, §7). Sampled from a third independent PRNG stream
+// (seed XOR FLOWER_PRNG_SALT) so the main stream stays bit-identical
+// to the pre-flower implementation. 4% of blades become flowers; each
+// flower has a head color (6-entry palette), head radius, and a stem
+// height bonus of 1.2x–1.5x. Non-flower blades carry heightBonus=1.0.
+constexpr double   FLOWER_PROBABILITY        = 0.04;
+constexpr double   FLOWER_HEIGHT_BONUS_MIN   = 1.2;
+constexpr double   FLOWER_HEIGHT_BONUS_MAX   = 1.5;
+constexpr double   FLOWER_HEAD_RADIUS_MIN    = 1.8;   // DIP
+constexpr double   FLOWER_HEAD_RADIUS_MAX    = 3.0;   // DIP
+constexpr int      FLOWER_PALETTE_SIZE       = 6;
+constexpr uint64_t FLOWER_PRNG_SALT          = 0xC0FFEEFACE0FFE5ull;
+
+constexpr uint32_t FLOWER_PALETTE[FLOWER_PALETTE_SIZE] = {
+    0xFFFFEB3Bu, // 0 yellow (dandelion)
+    0xFFFFA726u, // 1 orange (marigold)
+    0xFFFF80ABu, // 2 pink (cosmos)
+    0xFFE1BEE7u, // 3 lavender
+    0xFFFFFFFFu, // 4 white (daisy)
+    0xFFEF5350u, // 5 red (poppy)
+};
+
+// Mushrooms (PROTOTYPE — Native-only for now). 2.5% of blade slots become
+// mushrooms (filled-ellipse cap on a short stem). Sampled from a fourth
+// independent PRNG stream so adding mushrooms does NOT perturb the existing
+// flower / regrowth / main streams. Mushrooms preempt grass rendering at a
+// slot: the renderer draws the mushroom geometry and skips the grass blade
+// + flower head for that slot.
+constexpr double   MUSHROOM_PROBABILITY        = 0.025;
+constexpr double   MUSHROOM_CAP_WIDTH_MIN      = 4.0;   // DIP, radius X
+constexpr double   MUSHROOM_CAP_WIDTH_MAX      = 8.0;
+constexpr double   MUSHROOM_CAP_HEIGHT_MIN     = 2.5;   // DIP, radius Y (flatter than width)
+constexpr double   MUSHROOM_CAP_HEIGHT_MAX     = 5.0;
+constexpr double   MUSHROOM_STEM_HEIGHT_MIN    = 4.0;   // DIP
+constexpr double   MUSHROOM_STEM_HEIGHT_MAX    = 10.0;
+constexpr double   MUSHROOM_STEM_THICKNESS_MIN = 2.0;   // DIP
+constexpr double   MUSHROOM_STEM_THICKNESS_MAX = 4.0;
+constexpr int      MUSHROOM_PALETTE_SIZE       = 6;
+constexpr uint64_t MUSHROOM_PRNG_SALT          = 0xBADC0FFEE0FACE21ull;
+constexpr uint32_t MUSHROOM_STEM_COLOR         = 0xFFF5F5DCu; // beige/ivory
+
+// Ambient gusts (architecture.md §8.1). Small, randomly scheduled puffs of
+// wind that fire independently of cursor input. Implemented via a fifth
+// independent PRNG stream salted with AMBIENT_GUST_PRNG_SALT so adding
+// ambient gusts does NOT perturb the main / regrowth / flower / mushroom
+// streams — the §12 static blade snapshot is unchanged.
+//
+// Per-fire draw order (locked in §8.1): x, signDir, magFactor, interval.
+// Four draws per emitted puff, zero draws on idle ticks.
+constexpr uint64_t AMBIENT_GUST_PRNG_SALT       = 0xB7EE2EE2B7EE2EE2ull;
+constexpr double   AMBIENT_GUST_INTERVAL_MIN    = 5.0;   // sec
+constexpr double   AMBIENT_GUST_INTERVAL_MAX    = 15.0;  // sec
+constexpr double   AMBIENT_GUST_MAG_FACTOR_MIN  = 0.3;   // unitless, fraction of MAX_CURSOR_SPEED
+constexpr double   AMBIENT_GUST_MAG_FACTOR_MAX  = 0.6;
+constexpr double   AMBIENT_GUST_RADIUS_FACTOR   = 0.5;   // unitless, fraction of GUST_RADIUS
+
+// Desert scene shrinks non-cactus, non-mushroom blade heights at render
+// time so cacti read as the dominant biome feature.
+constexpr double   DESERT_GRASS_HEIGHT_SCALE     = 0.5;
+
+// Winter scene shrinks ordinary blade heights so pines and snow caps
+// read as the dominant features; mushrooms are also suppressed below.
+constexpr double   WINTER_GRASS_HEIGHT_SCALE     = 0.5;
+
+// Cacti (§14). Slot-bound Desert blade variants generated from an independent
+// PRNG stream so the §12 static blade snapshot remains unchanged.
+constexpr double   CACTUS_PROBABILITY            = 0.005;
+constexpr double   CACTUS_HEIGHT_MIN             = 30.0;
+constexpr double   CACTUS_HEIGHT_MAX             = 70.0;
+constexpr double   CACTUS_WIDTH_MIN              = 8.0;
+constexpr double   CACTUS_WIDTH_MAX              = 14.0;
+constexpr double   CACTUS_ARM_PROBABILITY        = 0.55;
+constexpr double   CACTUS_TWO_ARM_PROBABILITY    = 0.35;
+constexpr double   CACTUS_ARM_MIN_HEIGHT         = 50.0; // only tall cacti grow arms (range is 30-70)
+constexpr double   CACTUS_ARM_MIN_CUT_HEIGHT     = 0.85; // render-only: hide arms once a cactus is cut
+constexpr uint32_t CACTUS_COLOR                  = 0xFF2D7A2Du;
+constexpr uint64_t CACTUS_PRNG_SALT              = 0xCAC75CAC75CAC75Cull;
+
+// Tumbleweeds (§14). Desert roaming entities generated and respawned from a
+// persistent stream seeded with seed XOR TUMBLEWEED_PRNG_SALT.
+constexpr int      TUMBLEWEED_COUNT_PER_1920DIP  = 4;
+constexpr double   TUMBLEWEED_SIZE_MIN           = 8.0;
+constexpr double   TUMBLEWEED_SIZE_MAX           = 18.0;
+constexpr double   TUMBLEWEED_SPEED_MIN          = 24.0;
+constexpr double   TUMBLEWEED_SPEED_MAX          = 72.0;
+constexpr double   TUMBLEWEED_Y_OFFSET_MIN       = 8.0;
+constexpr double   TUMBLEWEED_Y_OFFSET_MAX       = 20.0;
+constexpr uint32_t TUMBLEWEED_COLOR              = 0xFF8A6A3Du;
+constexpr uint64_t TUMBLEWEED_PRNG_SALT          = 0x7B0117CA7B0117CAull;
+// Gentle, staggered vertical hop (§14). Heights are a fraction of the
+// tumbleweed radius so the bounce stays subtle; period is the rough gap
+// between hops, jittered per-hop. Gravity sets the arc/airtime.
+constexpr double   TUMBLEWEED_BOUNCE_GRAVITY     = 300.0;
+constexpr double   TUMBLEWEED_BOUNCE_HEIGHT_MIN_FRAC = 0.35;
+constexpr double   TUMBLEWEED_BOUNCE_HEIGHT_MAX_FRAC = 0.75;
+constexpr double   TUMBLEWEED_BOUNCE_PERIOD_MIN  = 2.5;
+constexpr double   TUMBLEWEED_BOUNCE_PERIOD_MAX  = 6.0;
+
+// Scenes (architecture.md §13). Render-time presentation modes that share
+// generation, sway, gust, cut, and ambient-gust logic. The infrastructure
+// pass swaps only the blade palette; per-scene entity content (cacti,
+// tumbleweeds, snowflakes, frost, falling leaves, maples) ships in §14/§15/§16.5.
+enum class Scene : uint8_t {
+    Grass  = 0,   // default
+    Desert = 1,
+    Winter = 2,
+    Autumn = 3,
+    Ocean  = 4,
+};
+constexpr int    SCENE_COUNT   = 5;
+constexpr Scene  SCENE_DEFAULT = Scene::Grass;
+
+// Per-scene blade palettes (§13). Each is six ARGB colors indexed by
+// blade.hue (drawn from the §5 main PRNG stream — generation is
+// scene-agnostic). The Grass palette is the original §4 PALETTE; the
+// Desert and Winter palettes are listed below.
+constexpr uint32_t DESERT_PALETTE[PALETTE_SIZE] = {
+    0xFFC9A26Bu,  // 0 dried-grass tan
+    0xFFB48A56u,  // 1 warm sand
+    0xFFD9B57Au,  // 2 light dune
+    0xFF8F6E3Fu,  // 3 dust brown
+    0xFFE6C896u,  // 4 pale beige
+    0xFFA67843u,  // 5 burnt sienna
+};
+
+constexpr uint32_t WINTER_PALETTE[PALETTE_SIZE] = {
+    0xFFE8EEF5u,  // 0 frost white
+    0xFFB7C4D2u,  // 1 cool silver
+    0xFFCBD8E5u,  // 2 pale ice
+    0xFFD7E2EEu,  // 3 light snow
+    0xFFA8B7C6u,  // 4 winter slate
+    0xFFEEF3F8u,  // 5 hoarfrost
+};
+
+constexpr uint32_t AUTUMN_PALETTE[PALETTE_SIZE] = {
+    0xFFD96B0Cu,  // 0 burnt orange
+    0xFFB54D1Eu,  // 1 deep rust
+    0xFFE89A3Cu,  // 2 warm amber
+    0xFFC23E12u,  // 3 vibrant red-orange
+    0xFFD9A65Cu,  // 4 honey-gold
+    0xFF8C2E0Fu,  // 5 dark maroon
+};
+
+// Ocean palette — seafloor sand / silt / pebble tones used for blades on
+// the Ocean scene (so non-coral grass slots read as wisps of seagrass on
+// a sandy bottom rather than green lawn).
+constexpr uint32_t OCEAN_PALETTE[PALETTE_SIZE] = {
+    0xFF3FA9A6u,  // 0 teal
+    0xFF2E8C8Au,  // 1 deep teal
+    0xFF6FC6C2u,  // 2 pale aqua
+    0xFF1F6F75u,  // 3 deep sea green
+    0xFF8FD7CCu,  // 4 light sea foam
+    0xFF257D7Bu,  // 5 mid teal
+};
+
+constexpr uint32_t MUSHROOM_PALETTE[MUSHROOM_PALETTE_SIZE] = {
+    0xFFD32F2Fu, // 0 red (amanita)
+    0xFF8D6E63u, // 1 brown
+    0xFFC9A66Bu, // 2 tan
+    0xFFFFF8E1u, // 3 ivory
+    0xFFE57373u, // 4 dusty pink
+    0xFF6D4C41u, // 5 dark brown
+};
+
+// Tests -----------------------------------------------------------------------
+constexpr uint64_t CANONICAL_TEST_SEED = 0x6B6173746Full;
+
+// ARGB palette. Alpha is always 0xFF; window-level transparency is at the
+// compositor.
+constexpr uint32_t PALETTE[PALETTE_SIZE] = {
+    0xFF2C5E1Au,
+    0xFF3A7A24u,
+    0xFF4C9A2Eu,
+    0xFF66B845u,
+    0xFF7AC957u,
+    0xFF8FD96Au,
+};
+
+// (§13) Per-scene blade palettes indexed by `[scene][hue]`. The Grass row
+// is the original §4 PALETTE, repeated for uniform indexing.
+constexpr uint32_t SCENE_PALETTES[SCENE_COUNT][PALETTE_SIZE] = {
+    { PALETTE[0],        PALETTE[1],        PALETTE[2],        PALETTE[3],        PALETTE[4],        PALETTE[5]        },
+    { DESERT_PALETTE[0], DESERT_PALETTE[1], DESERT_PALETTE[2], DESERT_PALETTE[3], DESERT_PALETTE[4], DESERT_PALETTE[5] },
+    { WINTER_PALETTE[0], WINTER_PALETTE[1], WINTER_PALETTE[2], WINTER_PALETTE[3], WINTER_PALETTE[4], WINTER_PALETTE[5] },
+    { AUTUMN_PALETTE[0], AUTUMN_PALETTE[1], AUTUMN_PALETTE[2], AUTUMN_PALETTE[3], AUTUMN_PALETTE[4], AUTUMN_PALETTE[5] },
+    { OCEAN_PALETTE[0],  OCEAN_PALETTE[1],  OCEAN_PALETTE[2],  OCEAN_PALETTE[3],  OCEAN_PALETTE[4],  OCEAN_PALETTE[5]  },
+};
+
+// Roaming-entity subsystem (§13.2). EntityKind discriminants are
+// cross-impl-locked. MAX_ENTITIES_PER_MONITOR caps the snowflake emitter
+// so the entities vector cannot grow without bound; the Sim pre-reserves
+// to this size at construction to avoid grow churn during the tick.
+enum class EntityKind : uint8_t {
+    None       = 0,
+    Tumbleweed = 1,
+    Snowflake  = 2,
+    Sheep      = 3,
+    Cat        = 4,
+    // 5 retired (Raindrop — rain effect removed); discriminant left as a gap
+    // so the remaining cross-impl-locked ordinals stay stable.
+    Bunny      = 6,
+    Butterfly  = 7,
+    Firefly    = 8,
+    Bird       = 9,
+    Hedgehog   = 10,
+    Leaf       = 11,
+    SnowPuff   = 12,
+    Bubble     = 13,
+    Fish       = 14,
+};
+constexpr int MAX_ENTITIES_PER_MONITOR = 64;
+
+// Critter subsystem — Grass-scene ambient critters plus legacy tray selectors.
+// CritterKind discriminants are cross-impl-locked.
+enum class CritterKind : uint8_t {
+    None  = 0,
+    Sheep = 1,
+    Cat   = 2,
+    Bunny = 3,
+};
+constexpr int         CRITTER_COUNT   = 4;
+constexpr CritterKind CRITTER_DEFAULT = CritterKind::None;
+constexpr uint64_t    CRITTER_PRNG_SALT = 0x5C8EE05C8EE05C8Eull;
+constexpr int         PET_COUNT_OPTIONS[] = { 1, 2, 3, 4, 5, 6 };
+constexpr int         PET_COUNT_DEFAULT_SHEEP = 2;
+constexpr int         PET_COUNT_DEFAULT_CAT = 1;
+constexpr int         PET_COUNT_MAX_PER_MONITOR = 6;
+constexpr const wchar_t* SHEEP_NAME_POOL[] = {
+    L"Bessie", L"Wooly", L"Clover", L"Daisy", L"Pippin", L"Buttercup", L"Mossy", L"Hazel"
+};
+constexpr const wchar_t* CAT_NAME_POOL[] = {
+    L"Mittens", L"Whiskers", L"Shadow", L"Ginger", L"Smokey", L"Boots", L"Sage", L"Juno"
+};
+constexpr const wchar_t* BUNNY_NAME_POOL[] = {
+    L"Clover", L"Hazel", L"Thumper", L"Mochi", L"Pip", L"Acorn",
+    L"Biscuit", L"Willow", L"Pepper", L"Hopper", L"Juniper", L"Snowdrop"
+};
+constexpr const wchar_t* HEDGEHOG_NAME_POOL[] = {
+    L"Bristle", L"Quill", L"Mossy", L"Truffle", L"Prickles", L"Snuffles",
+    L"Pinecone", L"Hazel", L"Bramble", L"Pip", L"Sage", L"Burdock"
+};
+constexpr double      PET_NAME_HOVER_RADIUS = 50.0;
+constexpr double      PET_NAME_FADE_DURATION = 1.5;
+constexpr double      PET_NAME_FONT_SIZE = 11.0;
+constexpr double      PET_NAME_OFFSET_Y = -8.0;
+constexpr uint32_t    PET_NAME_COLOR = 0xFFFFFFFFu;
+constexpr uint32_t    PET_NAME_SHADOW_COLOR = 0xC0000000u;
+
+// Sheep (§16). Procedurally drawn pet that walks, grazes, and idles along
+// the bottom strip. Phase 2: state machine (Walking / Grazing / Idle) with
+// animated leg cycle + head bob + grazing-head-down + idle-head-sweep.
+// Counts/speeds/sizes are sampled per-monitor from the critter PRNG so
+// different displays get different flocks. (Cursor-startle lands in §16.3.)
+constexpr int      SHEEP_COUNT_MIN     = 2;
+constexpr int      SHEEP_COUNT_MAX     = 3;
+constexpr double   SHEEP_WALK_SPEED_MIN = 14.0;   // DIP/sec
+constexpr double   SHEEP_WALK_SPEED_MAX = 26.0;
+
+// Geometry (DIP). Round, slightly tall body so the silhouette reads as
+// "cloud with legs" from a distance.
+constexpr double   SHEEP_BODY_RADIUS   = 12.0;    // body x-radius
+constexpr double   SHEEP_BODY_HEIGHT   = 9.5;     // body y-radius
+constexpr double   SHEEP_HEAD_RADIUS   = 5.0;
+constexpr double   SHEEP_LEG_LENGTH    = 5.5;
+constexpr double   SHEEP_TAIL_RADIUS   = 3.2;     // rear puff
+
+// Palette. Suffolk-style sheep: white wool, dark face, dark legs — that's
+// the silhouette people instantly read as "sheep". Cream/pink faces look
+// like a different creature entirely at this pixel scale.
+constexpr uint32_t SHEEP_BODY_COLOR    = 0xFFF7F4EBu;   // off-white wool
+constexpr uint32_t SHEEP_LEG_COLOR     = 0xFF1F1A16u;   // near-black
+constexpr uint32_t SHEEP_FACE_COLOR    = 0xFF1F1A16u;   // dark Suffolk face
+constexpr uint32_t SHEEP_EAR_COLOR     = 0xFF14110Eu;   // slightly darker than face
+constexpr uint32_t SHEEP_INK_COLOR     = 0xFFF7F4EBu;   // eyes = light dots on dark face
+
+// Animation cycle. WALK_PERIOD is one full leg cycle (one stride pair).
+constexpr double   SHEEP_WALK_PERIOD     = 0.55;        // seconds
+constexpr double   SHEEP_LEG_CYCLE_AMP   = 2.0;         // DIP vertical sway of leg-tip
+constexpr double   SHEEP_HEAD_BOB_AMP    = 0.7;         // DIP head Y bob during walk
+constexpr double   SHEEP_TAIL_WIGGLE_AMP = 0.6;         // DIP tail X wiggle
+
+// State machine. State encodes 0=Walking, 1=Grazing, 2=Idle, 3=Sleeping,
+// 4=Hopping, 5=Greeting in Entity.state. Walking → expires → Grazing /
+// Idle / Hopping. Idle → expires → Walking or Sleeping. Other states →
+// expires → Walking; Greeting flips vx on exit so paired sheep walk apart.
+// Durations drawn from critter PRNG on every transition so behavior is
+// deterministic per (seed, monitor). Click-near-sheep also forces Hopping.
+constexpr uint8_t  SHEEP_STATE_WALKING  = 0;
+constexpr uint8_t  SHEEP_STATE_GRAZING  = 1;
+constexpr uint8_t  SHEEP_STATE_IDLE     = 2;
+constexpr uint8_t  SHEEP_STATE_SLEEPING = 3;
+constexpr uint8_t  SHEEP_STATE_HOPPING  = 4;
+constexpr uint8_t  SHEEP_STATE_GREETING = 5;
+constexpr double   SHEEP_WALK_DURATION_MIN   = 8.0;     // sec — average walk leg before pause
+constexpr double   SHEEP_WALK_DURATION_MAX   = 14.0;
+constexpr double   SHEEP_GRAZE_DURATION_MIN  = 3.0;     // sec — head down chewing grass
+constexpr double   SHEEP_GRAZE_DURATION_MAX  = 5.0;
+constexpr double   SHEEP_IDLE_DURATION_MIN   = 1.5;     // sec — looking around
+constexpr double   SHEEP_IDLE_DURATION_MAX   = 3.0;
+constexpr double   SHEEP_SLEEP_DURATION_MIN  = 8.0;     // sec — Zzz nap
+constexpr double   SHEEP_SLEEP_DURATION_MAX  = 16.0;
+constexpr double   SHEEP_HOP_DURATION        = 0.55;    // sec — one parabolic arc
+constexpr double   SHEEP_GREET_RADIUS        = 50.0;    // DIP, center-to-center
+constexpr double   SHEEP_GREET_DURATION_MIN  = 1.6;     // sec
+constexpr double   SHEEP_GREET_DURATION_MAX  = 2.8;
+constexpr double   SHEEP_GREET_MIN_AGE       = 1.5;     // sec, natural cooldown
+constexpr double   SHEEP_CURIOUS_RADIUS      = 80.0;    // DIP, cursor proximity for noticing
+constexpr double   SHEEP_CURIOUS_HEAD_TURN_MAX = 0.55;  // radians, max head rotation toward cursor
+
+// Walking-expiry distribution. Cumulative: r<GRAZE → Grazing, else
+// r<GRAZE+IDLE → Idle, else → Hopping. GRAZE + IDLE + HOP_PROB == 1.0.
+constexpr double   SHEEP_GRAZE_PROBABILITY  = 0.60;
+constexpr double   SHEEP_IDLE_PROBABILITY   = 0.25;
+// Idle-expiry: chance of slipping into Sleeping vs returning to Walking.
+constexpr double   SHEEP_SLEEP_FROM_IDLE_PROB = 0.30;
+
+// Idle / Grazing / Greeting tiny animations.
+constexpr double   SHEEP_IDLE_SWEEP_FREQ      = 1.4;    // rad/sec for L/R head turn
+constexpr double   SHEEP_GRAZE_MUNCH_FREQ     = 8.0;    // rad/sec for head nibble bob
+constexpr double   SHEEP_GRAZE_MUNCH_AMP      = 0.6;    // DIP
+constexpr double   SHEEP_GREET_HEAD_BOB_FREQ  = 4.5;    // rad/sec
+constexpr double   SHEEP_GREET_HEAD_BOB_AMP   = 0.7;    // DIP, gentle nuzzle bob
+
+// Hop arc + click-startle.
+constexpr double   SHEEP_HOP_HEIGHT        = 11.0;      // DIP peak vertical offset
+constexpr double   SHEEP_STARTLE_RADIUS    = 64.0;      // DIP — click within this hops the sheep
+constexpr double   SHEEP_STARTLE_BOOST     = 1.6;       // walking speed multiplier post-startle
+
+// Sleeping cosmetic — "Zzz" glyphs drift up from the sheep's head.
+constexpr double   SHEEP_ZZZ_CYCLE_SEC     = 1.8;       // one Z lifespan
+constexpr double   SHEEP_ZZZ_RISE          = 11.0;      // DIP rise over one cycle
+constexpr double   SHEEP_ZZZ_SIZE_START    = 2.0;       // DIP starting Z side
+constexpr double   SHEEP_ZZZ_SIZE_END      = 4.0;       // DIP ending Z side
+
+// Cat (§17). Calm color-varied critter that reuses the sheep state byte values but
+// only uses Walking, Idle, Sleeping, and Hopping (semantically Pouncing).
+constexpr int      CAT_COUNT_MIN      = 1;
+constexpr int      CAT_COUNT_MAX      = 2;
+constexpr double   CAT_WALK_SPEED_MIN = 10.0;
+constexpr double   CAT_WALK_SPEED_MAX = 22.0;
+constexpr double   CAT_POUNCE_SPEED   = 60.0;
+
+constexpr double   CAT_BODY_RADIUS    = 11.0;
+constexpr double   CAT_BODY_HEIGHT    = 7.0;
+constexpr double   CAT_HEAD_RADIUS    = 4.5;
+constexpr double   CAT_LEG_LENGTH     = 5.0;
+constexpr double   CAT_TAIL_LENGTH    = 13.0;
+constexpr double   CAT_TAIL_THICKNESS = 1.6;
+constexpr double   CAT_EAR_HEIGHT     = 4.5;
+
+constexpr int CAT_COAT_VARIANT_COUNT = 6;
+
+struct CatCoatPalette {
+    uint32_t body;
+    uint32_t leg;
+    uint32_t face;
+    uint32_t ear;
+    uint32_t ink;
+};
+
+constexpr CatCoatPalette CAT_COAT_PALETTES[CAT_COAT_VARIANT_COUNT] = {
+    { 0xFF6B6259u, 0xFF3D3733u, 0xFF6B6259u, 0xFF3D3733u, 0xFF1A1614u },  // 0 Gray tabby (existing)
+    { 0xFFD89A6Fu, 0xFFA56B40u, 0xFFD89A6Fu, 0xFFA56B40u, 0xFF2B1A0Eu },  // 1 Orange
+    { 0xFF2A2522u, 0xFF140F0Cu, 0xFF2A2522u, 0xFF140F0Cu, 0xFFD9B85Bu },  // 2 Black (yellow eyes)
+    { 0xFFEDE9E1u, 0xFFBDB7ABu, 0xFFEDE9E1u, 0xFFBDB7ABu, 0xFF1F1817u },  // 3 White
+    { 0xFF7A5F3Cu, 0xFF4E3F26u, 0xFF7A5F3Cu, 0xFF4E3F26u, 0xFF1A1108u },  // 4 Brown tabby
+    { 0xFFC9B898u, 0xFF8E7F6Bu, 0xFFC9B898u, 0xFF8E7F6Bu, 0xFF2E251Du },  // 5 Cream
+};
+
+// Backward-compat aliases: variant 0 preserves the original muted gray tabby.
+constexpr uint32_t CAT_BODY_COLOR = CAT_COAT_PALETTES[0].body;
+constexpr uint32_t CAT_LEG_COLOR  = CAT_COAT_PALETTES[0].leg;
+constexpr uint32_t CAT_FACE_COLOR = CAT_COAT_PALETTES[0].face;
+constexpr uint32_t CAT_EAR_COLOR  = CAT_COAT_PALETTES[0].ear;
+constexpr uint32_t CAT_INK_COLOR  = CAT_COAT_PALETTES[0].ink;
+
+constexpr double   CAT_WALK_PERIOD    = 0.50;
+constexpr double   CAT_LEG_CYCLE_AMP  = 1.6;
+constexpr double   CAT_HEAD_BOB_AMP   = 0.4;
+constexpr double   CAT_TAIL_SWAY_FREQ = 1.2;
+constexpr double   CAT_TAIL_SWAY_AMP  = 0.35;
+
+constexpr uint8_t  CAT_STATE_WALKING  = SHEEP_STATE_WALKING;   // 0
+constexpr uint8_t  CAT_STATE_IDLE     = SHEEP_STATE_IDLE;      // 2, sit-and-watch
+constexpr uint8_t  CAT_STATE_SLEEPING = SHEEP_STATE_SLEEPING;  // 3
+constexpr uint8_t  CAT_STATE_POUNCING = SHEEP_STATE_HOPPING;   // 4, semantic alias
+
+constexpr double   CAT_WALK_DURATION_MIN  = 6.0;
+constexpr double   CAT_WALK_DURATION_MAX  = 10.0;
+constexpr double   CAT_IDLE_DURATION_MIN  = 4.0;
+constexpr double   CAT_IDLE_DURATION_MAX  = 8.0;
+constexpr double   CAT_SLEEP_DURATION_MIN = 20.0;
+constexpr double   CAT_SLEEP_DURATION_MAX = 40.0;
+constexpr double   CAT_POUNCE_DURATION    = 0.45;
+
+constexpr double   CAT_IDLE_PROBABILITY   = 0.65;
+constexpr double   CAT_SLEEP_PROBABILITY  = 0.30;
+constexpr double   CAT_SLEEP_FROM_IDLE_PROB = 0.50;
+
+constexpr double   CAT_POUNCE_RADIUS      = 80.0;
+constexpr double   CAT_POUNCE_HEIGHT      = 9.0;
+constexpr double   CAT_CURIOUS_RADIUS     = 100.0;
+constexpr double   CAT_CURIOUS_HEAD_TURN_MAX = 0.7;
+
+// Bunny (§18). Grass-only woodland critter: shy, passive, and always hopping
+// when it moves. Generated after sheep and cats from the shared critter PRNG.
+constexpr int      BUNNY_COUNT_MIN          = 1;
+constexpr int      BUNNY_COUNT_MAX          = 2;
+constexpr double   BUNNY_HOP_SPEED_MIN      = 22.0;
+constexpr double   BUNNY_HOP_SPEED_MAX      = 38.0;
+constexpr double   BUNNY_BODY_RADIUS        = 8.0;
+constexpr double   BUNNY_BODY_HEIGHT        = 6.5;
+constexpr double   BUNNY_HEAD_RADIUS        = 4.2;
+constexpr double   BUNNY_EAR_HEIGHT         = 9.0;
+constexpr double   BUNNY_EAR_WIDTH          = 2.2;
+constexpr double   BUNNY_EAR_SPACING        = 3.0;
+constexpr double   BUNNY_LEG_LENGTH         = 4.0;
+constexpr double   BUNNY_TAIL_RADIUS        = 2.4;
+constexpr uint32_t BUNNY_BODY_COLOR         = 0xFF8A6A4Au;
+constexpr uint32_t BUNNY_BELLY_COLOR        = 0xFFC4A98Du;
+constexpr uint32_t BUNNY_EAR_COLOR          = 0xFF8A6A4Au;
+constexpr uint32_t BUNNY_EAR_INNER_COLOR    = 0xFFD9A0A0u;
+constexpr uint32_t BUNNY_TAIL_COLOR         = 0xFFF7F4EBu;
+constexpr uint32_t BUNNY_EYE_COLOR          = 0xFF1A1208u;
+constexpr uint32_t BUNNY_NOSE_COLOR         = 0xFF8A4040u;
+
+constexpr uint8_t  BUNNY_STATE_HOPPING      = 0;
+constexpr uint8_t  BUNNY_STATE_GRAZING      = 1;
+constexpr uint8_t  BUNNY_STATE_IDLE         = 2;
+constexpr uint8_t  BUNNY_STATE_SLEEPING     = 3;
+constexpr uint8_t  BUNNY_STATE_STARTLED     = 4;
+
+constexpr double   BUNNY_HOP_DURATION       = 0.40;
+constexpr double   BUNNY_HOP_HEIGHT         = 8.0;
+constexpr double   BUNNY_HOP_GAP_MIN        = 0.05;
+constexpr double   BUNNY_HOP_GAP_MAX        = 0.20;
+constexpr double   BUNNY_GRAZE_DURATION_MIN = 2.5;
+constexpr double   BUNNY_GRAZE_DURATION_MAX = 4.5;
+constexpr double   BUNNY_IDLE_DURATION_MIN  = 2.0;
+constexpr double   BUNNY_IDLE_DURATION_MAX  = 4.0;
+constexpr double   BUNNY_SLEEP_DURATION_MIN = 6.0;
+constexpr double   BUNNY_SLEEP_DURATION_MAX = 12.0;
+constexpr double   BUNNY_GRAZE_PROBABILITY  = 0.55;
+constexpr double   BUNNY_IDLE_PROBABILITY   = 0.30;
+constexpr double   BUNNY_SLEEP_PROB         = 0.05;
+
+constexpr double   BUNNY_STARTLE_RADIUS     = 90.0;
+constexpr double   BUNNY_STARTLE_BOOST      = 2.0;
+constexpr double   BUNNY_STARTLE_HOP_HEIGHT = 14.0;
+constexpr double   BUNNY_STARTLE_DURATION   = 3.0;
+
+constexpr double   BUNNY_NOSE_TWITCH_FREQ   = 6.0;
+constexpr double   BUNNY_NOSE_TWITCH_AMP    = 0.5;
+constexpr double   BUNNY_EAR_WIGGLE_FREQ    = 1.2;
+constexpr double   BUNNY_EAR_WIGGLE_AMP     = 0.20;
+
+constexpr double   BUNNY_ZZZ_CYCLE_SEC      = SHEEP_ZZZ_CYCLE_SEC;
+constexpr double   BUNNY_ZZZ_RISE           = SHEEP_ZZZ_RISE * 0.7;
+constexpr double   BUNNY_ZZZ_SIZE_START     = SHEEP_ZZZ_SIZE_START * 0.7;
+constexpr double   BUNNY_ZZZ_SIZE_END       = SHEEP_ZZZ_SIZE_END * 0.7;
+// Hedgehog (§17.9). Grass-only, solitary nocturnal critter. Generated after
+// bunnies from the shared critter PRNG; passive defense curls into a ball.
+constexpr int      HEDGEHOG_COUNT_MIN             = 0;
+constexpr int      HEDGEHOG_COUNT_MAX             = 1;
+constexpr double   HEDGEHOG_COUNT_PROBABILITY     = 0.55;
+constexpr double   HEDGEHOG_WALK_SPEED_MIN        = 4.0;
+constexpr double   HEDGEHOG_WALK_SPEED_MAX        = 8.0;
+constexpr double   HEDGEHOG_BODY_RADIUS           = 9.0;
+constexpr double   HEDGEHOG_BODY_HEIGHT           = 5.5;
+constexpr double   HEDGEHOG_HEAD_RADIUS           = 3.6;
+constexpr double   HEDGEHOG_NOSE_RADIUS           = 0.8;
+constexpr double   HEDGEHOG_LEG_LENGTH            = 2.5;
+constexpr int      HEDGEHOG_SPIKE_COUNT           = 14;
+constexpr double   HEDGEHOG_SPIKE_LENGTH          = 3.0;
+constexpr double   HEDGEHOG_SPIKE_WIDTH           = 1.4;
+constexpr double   HEDGEHOG_SPIKE_ARC_START_DEG   = -20.0;
+constexpr double   HEDGEHOG_SPIKE_ARC_END_DEG     = 200.0;
+constexpr uint32_t HEDGEHOG_BODY_COLOR            = 0xFF5C4633u;
+constexpr uint32_t HEDGEHOG_SPIKE_COLOR           = 0xFF3A2A1Fu;
+constexpr uint32_t HEDGEHOG_SPIKE_TIP_COLOR       = 0xFF1E150Eu;
+constexpr uint32_t HEDGEHOG_NOSE_COLOR            = 0xFF1A1208u;
+constexpr uint32_t HEDGEHOG_EYE_COLOR             = 0xFF1A1208u;
+
+constexpr uint8_t  HEDGEHOG_STATE_WALKING         = 0;
+constexpr uint8_t  HEDGEHOG_STATE_SNUFFLING       = 1;
+constexpr uint8_t  HEDGEHOG_STATE_IDLE            = 2;
+constexpr uint8_t  HEDGEHOG_STATE_SLEEPING        = 3;
+constexpr uint8_t  HEDGEHOG_STATE_CURLED          = 4;
+
+constexpr double   HEDGEHOG_WALK_DURATION_MIN     = 6.0;
+constexpr double   HEDGEHOG_WALK_DURATION_MAX     = 12.0;
+constexpr double   HEDGEHOG_SNUFFLE_DURATION_MIN  = 3.0;
+constexpr double   HEDGEHOG_SNUFFLE_DURATION_MAX  = 6.0;
+constexpr double   HEDGEHOG_IDLE_DURATION_MIN     = 1.5;
+constexpr double   HEDGEHOG_IDLE_DURATION_MAX     = 3.0;
+constexpr double   HEDGEHOG_SLEEP_DURATION_MIN    = 10.0;
+constexpr double   HEDGEHOG_SLEEP_DURATION_MAX    = 25.0;
+constexpr double   HEDGEHOG_CURL_DURATION_MIN     = 3.0;
+constexpr double   HEDGEHOG_CURL_DURATION_MAX     = 5.5;
+constexpr double   HEDGEHOG_SNUFFLE_PROBABILITY   = 0.55;
+constexpr double   HEDGEHOG_IDLE_PROBABILITY      = 0.30;
+constexpr double   HEDGEHOG_SLEEP_PROB            = 0.50;
+constexpr double   HEDGEHOG_STARTLE_RADIUS        = 70.0;
+constexpr double   HEDGEHOG_SNUFFLE_HEAD_FREQ     = 5.0;
+constexpr double   HEDGEHOG_SNUFFLE_HEAD_AMP      = 0.7;
+constexpr double   HEDGEHOG_WADDLE_FREQ           = 4.0;
+constexpr double   HEDGEHOG_WADDLE_AMP            = 0.8;
+constexpr double   HEDGEHOG_ZZZ_CYCLE_SEC         = SHEEP_ZZZ_CYCLE_SEC;
+constexpr double   HEDGEHOG_ZZZ_RISE              = SHEEP_ZZZ_RISE * 0.5;
+constexpr double   HEDGEHOG_ZZZ_SIZE_START        = SHEEP_ZZZ_SIZE_START * 0.6;
+constexpr double   HEDGEHOG_ZZZ_SIZE_END          = SHEEP_ZZZ_SIZE_END * 0.6;
+// Butterflies (§17.6). Grass-only, passive daytime ambient flyers.
+
+constexpr int      BUTTERFLY_COUNT_MIN          = 2;
+constexpr int      BUTTERFLY_COUNT_MAX          = 4;
+constexpr double   BUTTERFLY_SPEED_MIN          = 18.0;
+constexpr double   BUTTERFLY_SPEED_MAX          = 32.0;
+constexpr double   BUTTERFLY_BODY_LENGTH        = 2.4;
+constexpr double   BUTTERFLY_WING_RADIUS        = 3.5;
+constexpr double   BUTTERFLY_WING_OFFSET        = 2.2;
+constexpr double   BUTTERFLY_FLUTTER_FREQ       = 16.0;
+constexpr double   BUTTERFLY_FLUTTER_MIN_SCALE  = 0.20;
+constexpr double   BUTTERFLY_MEANDER_FREQ_Y     = 0.8;
+constexpr double   BUTTERFLY_MEANDER_AMP_Y      = 16.0;
+constexpr double   BUTTERFLY_MEANDER_FREQ_X     = 0.5;
+constexpr double   BUTTERFLY_MEANDER_AMP_X      = 0.4;
+constexpr double   BUTTERFLY_ALTITUDE_MIN       = 18.0;
+constexpr double   BUTTERFLY_ALTITUDE_MAX       = 70.0;
+constexpr uint32_t BUTTERFLY_BODY_COLOR         = 0xFF2A2018u;
+constexpr int      BUTTERFLY_COLOR_COUNT        = 5;
+constexpr uint64_t BUTTERFLY_PRNG_SALT          = 0xB07DEF1E0001ull;
+
+struct ButterflyPalette {
+    uint32_t wingColor;
+    uint32_t accentColor;
+};
+
+constexpr ButterflyPalette BUTTERFLY_PALETTES[BUTTERFLY_COLOR_COUNT] = {
+    { 0xFFFF9A2Eu, 0xFF1A130Cu }, // 0 Monarch: orange + black tips
+    { 0xFFFFD34Du, 0xFF1A130Cu }, // 1 Swallowtail: yellow + black
+    { 0xFFFFF8E8u, 0xFF3A3A3Au }, // 2 Cabbage: white + dark dots
+    { 0xFF63C7FFu, 0xFF1B4D99u }, // 3 Morpho: sky blue + deeper blue
+    { 0xFFFFA6C8u, 0xFFFF6EA8u }, // 4 Pink: soft pink + rose
+};
+
+// Fireflies (§17.7). Grass-only, passive ambient flyers.
+constexpr int      FIREFLY_COUNT_MIN            = 3;
+constexpr int      FIREFLY_COUNT_MAX            = 6;
+constexpr double   FIREFLY_DRIFT_SPEED_MIN      = 4.0;
+constexpr double   FIREFLY_DRIFT_SPEED_MAX      = 10.0;
+constexpr double   FIREFLY_BODY_RADIUS          = 1.2;
+constexpr double   FIREFLY_GLOW_RADIUS          = 5.0;
+constexpr double   FIREFLY_BLINK_PERIOD_MIN     = 1.4;
+constexpr double   FIREFLY_BLINK_PERIOD_MAX     = 2.6;
+constexpr double   FIREFLY_BLINK_DUTY           = 0.55;
+constexpr double   FIREFLY_BLINK_FADE           = 0.30;
+constexpr double   FIREFLY_DRIFT_FREQ_X         = 0.4;
+constexpr double   FIREFLY_DRIFT_FREQ_Y         = 0.6;
+constexpr double   FIREFLY_DRIFT_AMP_X          = 0.6;
+constexpr double   FIREFLY_DRIFT_AMP_Y          = 8.0;
+constexpr double   FIREFLY_ALTITUDE_MIN         = 8.0;
+constexpr double   FIREFLY_ALTITUDE_MAX         = 55.0;
+constexpr uint32_t FIREFLY_BODY_COLOR           = 0xFFFFEE88u;
+constexpr uint32_t FIREFLY_GLOW_COLOR_RGB       = 0xEEDD66u;
+constexpr int      FIREFLY_GLOW_ALPHA_MAX       = 110;
+constexpr int      FIREFLY_BODY_ALPHA_MAX       = 255;
+constexpr uint64_t FIREFLY_PRNG_SALT            = 0xF13EF1E7777ull;
+
+// Bird flybys (§17.8). Grass-only transient flocks.
+constexpr double   BIRD_FLYBY_SPAWN_RATE_PER_HOUR = 15.0;
+constexpr int      BIRD_FLOCK_SIZE_MIN             = 3;
+constexpr int      BIRD_FLOCK_SIZE_MAX             = 7;
+constexpr double   BIRD_FLOCK_FORMATION_SPACING    = 9.0;
+constexpr double   BIRD_FLOCK_V_ANGLE_DEG          = 22.0;
+constexpr double   BIRD_SPEED_MIN                  = 65.0;
+constexpr double   BIRD_SPEED_MAX                  = 95.0;
+constexpr double   BIRD_ALTITUDE_MIN               = 78.0;
+constexpr double   BIRD_ALTITUDE_MAX               = 96.0;
+constexpr double   BIRD_BODY_LENGTH                = 3.6;
+constexpr double   BIRD_WING_SPAN                  = 5.0;
+constexpr double   BIRD_WING_FLAP_FREQ             = 7.0;
+constexpr double   BIRD_WING_FLAP_PHASE_JITTER     = 0.6;
+constexpr uint32_t BIRD_BODY_COLOR                 = 0xFF1A1610u;
+constexpr double   BIRD_WING_OPEN_RATIO            = 1.0;
+constexpr double   BIRD_WING_FOLD_RATIO            = 0.30;
+constexpr double   BIRD_FADE_IN_FRAC               = 0.08;
+constexpr double   BIRD_FADE_OUT_FRAC              = 0.08;
+constexpr double   BIRD_DRIFT_AMP_Y                = 3.0;
+constexpr double   BIRD_DRIFT_FREQ_Y               = 0.8;
+constexpr uint64_t BIRD_FLYBY_PRNG_SALT            = 0xB12D1F1A1B12D1Aull;
+
+// Snowflakes (§15)
+constexpr double   SNOWFLAKE_EMIT_RATE_PER_1920DIP = 8.0;    // flakes/sec
+constexpr double   SNOWFLAKE_FALL_SPEED_MIN        = 20.0;   // DIP/sec
+constexpr double   SNOWFLAKE_FALL_SPEED_MAX        = 40.0;
+constexpr double   SNOWFLAKE_SIZE_MIN              = 1.5;    // DIP
+constexpr double   SNOWFLAKE_SIZE_MAX              = 3.0;
+constexpr double   SNOWFLAKE_SWAY_AMPLITUDE        = 10.0;   // DIP
+constexpr double   SNOWFLAKE_SWAY_FREQUENCY        = 0.6;    // Hz
+constexpr double   SNOWFLAKE_LIFETIME_PADDING_SEC  = 2.0;
+constexpr uint32_t SNOWFLAKE_COLOR                 = 0xFFFFFFFFu;
+constexpr uint64_t SNOWFLAKE_PRNG_SALT             = 0xC0FFEE1CECAFEBABull;
+
+// Snow puff (§21). A click in the Winter scene kicks up a short-lived burst
+// of powder. Dedicated PRNG stream (salted) so the burst never perturbs the
+// snowflake emitter; it only fires on click input. y is screen-down, so an
+// upward launch is negative vy and SNOW_PUFF_GRAVITY pulls back toward ground.
+constexpr int      SNOW_PUFF_COUNT_MIN          = 9;
+constexpr int      SNOW_PUFF_COUNT_MAX          = 16;
+constexpr double   SNOW_PUFF_SIZE_MIN           = 3.5;    // DIP
+constexpr double   SNOW_PUFF_SIZE_MAX           = 8.0;
+constexpr double   SNOW_PUFF_BURST_SPEED_MIN    = 70.0;   // DIP/sec
+constexpr double   SNOW_PUFF_BURST_SPEED_MAX    = 150.0;
+constexpr double   SNOW_PUFF_SPREAD_RAD         = 1.25;   // half-angle about vertical
+constexpr double   SNOW_PUFF_GRAVITY            = 150.0;  // DIP/sec^2
+constexpr double   SNOW_PUFF_DRAG               = 1.6;    // horizontal decay
+constexpr double   SNOW_PUFF_START_RADIUS       = 7.0;    // initial scatter around click
+constexpr double   SNOW_PUFF_LIFETIME_MIN       = 1.0;    // sec
+constexpr double   SNOW_PUFF_LIFETIME_MAX       = 1.8;
+// Puffs are white, so on the white bank they need an edge: the cool bank-shadow
+// brush is reused to draw a slightly larger disc offset down behind the core.
+constexpr double   SNOW_PUFF_SHADOW_SCALE       = 1.35;   // shadow radius vs core
+constexpr double   SNOW_PUFF_SHADOW_OFFSET      = 0.45;   // downward offset vs core radius
+constexpr double   SNOW_PUFF_SHADOW_OPACITY     = 0.55;   // relative to the puff's age alpha
+constexpr uint64_t SNOW_PUFF_PRNG_SALT          = 0x5503FF1E5503FF1Eull;
+
+// §21.1 Snow drift (Winter cursor-move spindrift). Brushing the cursor low and
+// fast across the snowbank kicks up a small, gentle wisp of powder — the Winter
+// analogue of the autumn leaf-puff hover, giving the scene a calm move-driven
+// interaction to match grass/desert/fall. Reuses the snow-puff particle but with
+// fewer, smaller, slower grains. A global cooldown keeps it from spamming.
+constexpr int      SNOW_DRIFT_COUNT_MIN         = 4;
+constexpr int      SNOW_DRIFT_COUNT_MAX         = 8;
+constexpr double   SNOW_DRIFT_REACH_DIP         = 70.0;   // cursor must be this near the ground
+constexpr double   SNOW_DRIFT_MIN_SPEED         = 90.0;   // DIP/sec; only kicks up while moving
+constexpr double   SNOW_DRIFT_COOLDOWN_SEC      = 0.12;   // global gate (~8 wisps/sec max)
+constexpr double   SNOW_DRIFT_SIZE_SCALE        = 0.9;    // a touch smaller than a click burst
+constexpr double   SNOW_DRIFT_SPEED_SCALE       = 0.85;   // a touch gentler kick
+constexpr uint64_t SNOW_DRIFT_PRNG_SALT         = 0x5D81F77D5D81F77Dull;
+
+// Cool shadow tint reused by the live snow-puff to give white powder an edge
+// against light backgrounds (despite the legacy "bank" name).
+constexpr uint32_t SNOW_BANK_SHADOW_COLOR       = 0xFFBFCDE4u; // cool blue base/trough shadow
+
+// Falling leaves (§16.5). Autumn-only transient particles.
+constexpr double   LEAF_SPAWN_RATE_PER_SEC_1920DIP = 1.4;
+constexpr double   LEAF_FALL_SPEED_MIN             = 14.0;
+constexpr double   LEAF_FALL_SPEED_MAX             = 26.0;
+constexpr double   LEAF_HORIZONTAL_DRIFT_AMP       = 32.0;
+constexpr double   LEAF_HORIZONTAL_DRIFT_FREQ      = 1.4;
+constexpr double   LEAF_ROTATION_SPEED_MIN         = 0.8;
+constexpr double   LEAF_ROTATION_SPEED_MAX         = 2.4;
+constexpr double   LEAF_SIZE_MIN                   = 4.0;
+constexpr double   LEAF_SIZE_MAX                   = 7.0;
+constexpr double   LEAF_SPAWN_Y_OFFSET             = -10.0;
+constexpr int      LEAF_COLOR_COUNT                = 6;
+constexpr uint32_t LEAF_COLOR_0                    = 0xFFD96B0Cu;
+constexpr uint32_t LEAF_COLOR_1                    = 0xFFB54D1Eu;
+constexpr uint32_t LEAF_COLOR_2                    = 0xFFE89A3Cu;
+constexpr uint32_t LEAF_COLOR_3                    = 0xFFC23E12u;
+constexpr uint32_t LEAF_COLOR_4                    = 0xFFE6C849u;
+constexpr uint32_t LEAF_COLOR_5                    = 0xFF8C2E0Fu;
+constexpr uint64_t LEAF_PRNG_SALT                  = 0x1EA1DEC1D1EA1D05ull;
+constexpr uint32_t LEAF_COLORS[LEAF_COLOR_COUNT] = {
+    LEAF_COLOR_0, LEAF_COLOR_1, LEAF_COLOR_2,
+    LEAF_COLOR_3, LEAF_COLOR_4, LEAF_COLOR_5,
+};
+
+// Snow-tipped blade caps (§15)
+constexpr double   SNOW_TIP_RADIUS_FACTOR          = 1.25;
+constexpr uint32_t SNOW_TIP_COLOR                  = 0xFFFFFFFFu;
+
+// Pine trees (§15.1). Winter biome anchor — slot-bound, mirrors §14 cacti.
+constexpr double   PINE_PROBABILITY                = 0.0075;
+constexpr double   PINE_HEIGHT_MIN                 = 45.0;
+constexpr double   PINE_HEIGHT_MAX                 = 90.0;
+constexpr double   PINE_WIDTH_MIN                  = 28.0;
+constexpr double   PINE_WIDTH_MAX                  = 48.0;
+constexpr int      PINE_TIER_COUNT_MIN             = 2;
+constexpr int      PINE_TIER_COUNT_MAX             = 4;
+constexpr double   PINE_TIP_TAPER                  = 0.25;
+constexpr double   PINE_TIER_OVERLAP               = 0.15;
+constexpr double   PINE_SNOW_CAP_FRACTION          = 0.30;
+constexpr uint32_t PINE_COLOR                      = 0xFF1B5E20u;
+// Dimensional shading for pine boughs: a darker green dropped down-right as a
+// self-shadow and a lighter green dabbed on the upper-left lit face, so each
+// tier reads as a rounded bough instead of a flat triangle.
+constexpr uint32_t PINE_SHADOW_COLOR               = 0xFF103D16u;
+constexpr uint32_t PINE_HIGHLIGHT_COLOR            = 0xFF43A047u;
+constexpr double   PINE_SHADOW_OFFSET_X_FRAC       = 0.14;
+constexpr double   PINE_SHADOW_OFFSET_Y_FRAC       = 0.07;
+constexpr double   PINE_HIGHLIGHT_OFFSET_X_FRAC    = 0.20;
+constexpr double   PINE_HIGHLIGHT_WIDTH_FRAC       = 0.50;
+constexpr float    PINE_HIGHLIGHT_OPACITY          = 0.5f;
+constexpr uint64_t PINE_PRNG_SALT                  = 0x50494E4550494E45ull;
+
+// Tree sway (§15.2). Fall maples and winter pines/birches are blades, so they
+// already carry effectiveLean (ambient sway + nearby-cursor gusts). The renderer
+// shears each tree about its trunk base by a fraction of that lean so the canopy
+// drifts ever so slightly with the wind and the mouse — the same life the grass
+// has, scaled way down. TREE_SWAY_LEAN_FACTOR damps the grass-calibrated lean;
+// TREE_SWAY_MAX_HEIGHT_FRACTION clamps the apex shift to a fraction of tree
+// height so a hard gust can never visibly snap the trunk.
+constexpr double   TREE_SWAY_LEAN_FACTOR           = 0.6;
+constexpr double   TREE_SWAY_MAX_HEIGHT_FRACTION   = 0.05;
+
+// Tree depth layering (§15.4). Winter pines/birches are split into a foreground
+// layer (full size, drawn in front of the snowbank) and a background layer
+// (scaled down, hazier, drawn behind the snowbank) so the treeline reads with
+// real fore/background depth instead of a single flat row. The depth is chosen
+// by one locked PRNG draw per tree at generation time. Render-only scale/opacity.
+constexpr double   TREE_BACKGROUND_PROBABILITY     = 0.45;  // share of trees pushed to the back
+constexpr double   TREE_BG_SCALE                   = 0.62;  // background trees are ~62% size
+constexpr float    TREE_BG_OPACITY                 = 0.78f; // atmospheric haze on background trees
+
+// Minimum visible gap between two adjacent props of the same kind/layer
+// (pine/birch, cactus, maple, coral). Without this, the per-blade probability
+// roll can place two props directly on top of each other in tight clusters.
+// Each generator computes an effective collision half-width per prop and
+// enforces `nextLeftEdge >= prevRightEdge + PROP_MIN_GAP_DIP`. Background
+// vs foreground pines are tracked independently — they render at different
+// z-depths and overlap there is intentional parallax, not visual crowding.
+constexpr double   PROP_MIN_GAP_DIP                = 4.0;
+
+// Birch tree variant (§15.1). Second tree style — vertical white trunk
+// with dark bark marks and short bare branches. Selected per-slot via
+// an additional PRNG draw on tree promotion.
+constexpr double   BIRCH_VARIANT_PROBABILITY       = 0.30;
+constexpr double   BIRCH_TRUNK_WIDTH_MIN           = 4.0;   // DIP
+constexpr double   BIRCH_TRUNK_WIDTH_MAX           = 7.0;   // DIP
+constexpr int      BIRCH_BARK_MARK_COUNT           = 5;     // short centered horizontal dashes
+constexpr double   BIRCH_BARK_MARK_LENGTH_FRAC     = 0.50;  // max fraction of trunk width
+constexpr int      BIRCH_BRANCH_COUNT              = 6;     // upward-angled branches with snow tips
+constexpr double   BIRCH_SNOW_CAP_FRACTION         = 0.18;  // fraction of trunk height
+constexpr uint32_t BIRCH_BARK_COLOR                = 0xFFEFEFE6u; // off-white trunk
+constexpr uint32_t BIRCH_MARK_COLOR                = 0xFF2A2A28u; // dark bark stripes
+
+// Maple trees (§16.5). Autumn slot-bound biome anchor, shorter and warmer than pines.
+constexpr double   MAPLE_PROBABILITY               = 0.0070;
+constexpr double   MAPLE_HEIGHT_MIN                = 50.0;
+constexpr double   MAPLE_HEIGHT_MAX                = 85.0;
+constexpr double   MAPLE_TRUNK_WIDTH_MIN           = 6.0;
+constexpr double   MAPLE_TRUNK_WIDTH_MAX           = 10.0;
+constexpr double   MAPLE_CANOPY_RADIUS_MIN         = 14.0;
+constexpr double   MAPLE_CANOPY_RADIUS_MAX         = 24.0;
+constexpr uint32_t MAPLE_TRUNK_COLOR               = 0xFF4A2C18u;
+constexpr uint32_t MAPLE_TRUNK_DARK                = 0xFF2F1B0Eu;
+constexpr int      MAPLE_CANOPY_COLOR_COUNT        = 4;
+constexpr uint32_t MAPLE_CANOPY_COLOR_0            = 0xFFD96B0Cu;
+constexpr uint32_t MAPLE_CANOPY_COLOR_1            = 0xFFE89A3Cu;
+constexpr uint32_t MAPLE_CANOPY_COLOR_2            = 0xFFC23E12u;
+constexpr uint32_t MAPLE_CANOPY_COLOR_3            = 0xFFE6C849u;
+constexpr double   MAPLE_BARE_FRACTION             = 0.20;
+constexpr uint64_t MAPLE_PRNG_SALT                 = 0xC1AA51EC1AA51Eull;
+constexpr uint32_t MAPLE_CANOPY_COLORS[MAPLE_CANOPY_COLOR_COUNT] = {
+    MAPLE_CANOPY_COLOR_0, MAPLE_CANOPY_COLOR_1,
+    MAPLE_CANOPY_COLOR_2, MAPLE_CANOPY_COLOR_3,
+};
+
+// Leaf puff (§16.6). Hovering the cursor over a leafy maple canopy in Autumn
+// shakes a small flurry of leaves loose, like a gust caught the crown. Each
+// puff draws from an independent salted PRNG stream so it never perturbs the
+// ambient leaf emitter. A per-tree cooldown keeps re-hovers calm. Puff leaves
+// reuse the ordinary Leaf entity but carry an outward burst velocity (vx) that
+// decays via LEAF_PUFF_DRAG before they settle into the usual flutter-down.
+constexpr int      LEAF_PUFF_COUNT_MIN          = 4;
+constexpr int      LEAF_PUFF_COUNT_MAX          = 7;
+constexpr double   LEAF_PUFF_BURST_SPEED_MIN    = 18.0;   // DIP/s outward
+constexpr double   LEAF_PUFF_BURST_SPEED_MAX    = 42.0;   // DIP/s outward
+constexpr double   LEAF_PUFF_DRAG               = 2.2;    // exp decay rate (1/s) on burst vx
+constexpr double   LEAF_PUFF_COOLDOWN_SEC       = 1.5;    // per-tree re-puff gate
+constexpr double   LEAF_PUFF_HOVER_RADIUS_MUL   = 1.15;   // × canopy radius
+constexpr double   LEAF_PUFF_MIN_CUT_HEIGHT     = 0.5;    // tree must be reasonably leafy
+constexpr double   LEAF_PUFF_START_OFFSET_FRAC  = 0.4;    // spawn spread within canopy
+constexpr uint64_t LEAF_PUFF_PRNG_SALT          = 0x9E3779B97F4A7C15ull;
+
+// Ocean scene — coral (blade variant), bubbles (rising entity), fish
+// (horizontal swimmer). Coral probability is intentionally lower than
+// pines/maples because each piece is wider (multi-DIP fan/brain).
+constexpr double   CORAL_PROBABILITY     = 0.018;
+constexpr double   CORAL_HEIGHT_MIN      = 22.0;
+constexpr double   CORAL_HEIGHT_MAX      = 48.0;
+constexpr double   CORAL_WIDTH_MIN       = 10.0;
+constexpr double   CORAL_WIDTH_MAX       = 20.0;
+constexpr int      CORAL_TYPE_COUNT      = 3;   // 0 = fan, 1 = branching, 2 = brain
+constexpr int      CORAL_COLOR_COUNT     = 5;
+constexpr uint32_t CORAL_COLOR_0         = 0xFFFF6FA8u; // pink
+constexpr uint32_t CORAL_COLOR_1         = 0xFFFF8A3Du; // orange
+constexpr uint32_t CORAL_COLOR_2         = 0xFFB155D9u; // purple
+constexpr uint32_t CORAL_COLOR_3         = 0xFFE53935u; // red
+constexpr uint32_t CORAL_COLOR_4         = 0xFFFFE6D0u; // bone-white
+constexpr uint32_t CORAL_COLORS[CORAL_COLOR_COUNT] = {
+    CORAL_COLOR_0, CORAL_COLOR_1, CORAL_COLOR_2, CORAL_COLOR_3, CORAL_COLOR_4,
+};
+constexpr uint64_t CORAL_PRNG_SALT       = 0xC04A1C04A1C04A1Cull;
+
+// Bubbles — rise from the seafloor with horizontal wobble, pop at the top
+// of the canvas. Emit rate mirrors snowflake but at a calmer cadence.
+constexpr double   BUBBLE_EMIT_RATE_PER_1920DIP = 1.8;
+constexpr double   BUBBLE_RISE_SPEED_MIN   = 18.0;
+constexpr double   BUBBLE_RISE_SPEED_MAX   = 38.0;
+constexpr double   BUBBLE_SIZE_MIN         = 2.0;
+constexpr double   BUBBLE_SIZE_MAX         = 4.5;
+constexpr double   BUBBLE_WOBBLE_AMPLITUDE = 6.0;
+constexpr double   BUBBLE_WOBBLE_FREQUENCY = 0.7;
+constexpr double   BUBBLE_LIFETIME_PADDING_SEC = 1.5;
+constexpr uint32_t BUBBLE_STROKE_COLOR     = 0xCCB0E4FFu;
+constexpr uint32_t BUBBLE_HIGHLIGHT_COLOR  = 0xFFFFFFFFu;
+constexpr uint64_t BUBBLE_PRNG_SALT        = 0xB0BB1EB0BB1EB0BBull;
+
+// Fish — small swimmers confined to the visible strip so they stay on
+// canvas. The overlay is only STRIP_HEIGHT + HEADROOM (≈110 DIP) tall,
+// so altitudes are tight: 25..75 DIP above the ground line.
+constexpr double   FISH_COUNT_PER_1920DIP = 2.5;
+constexpr int      FISH_COUNT_MIN         = 2;
+constexpr int      FISH_COUNT_MAX         = 8;
+constexpr double   FISH_SPEED_MIN         = 18.0;
+constexpr double   FISH_SPEED_MAX         = 38.0;
+constexpr double   FISH_SIZE_MIN          = 5.0;   // body half-length DIP
+constexpr double   FISH_SIZE_MAX          = 8.5;
+constexpr double   FISH_ALTITUDE_MIN      = 25.0;  // DIP above ground
+constexpr double   FISH_ALTITUDE_MAX      = 75.0;
+constexpr double   FISH_TAIL_WOBBLE_FREQ  = 6.0;
+constexpr double   FISH_TAIL_WOBBLE_AMP   = 0.45;  // radians
+constexpr int      FISH_COLOR_COUNT       = 4;
+constexpr uint32_t FISH_COLOR_0           = 0xFFFFA844u; // clownfish orange
+constexpr uint32_t FISH_COLOR_1           = 0xFFFFD54Fu; // yellow
+constexpr uint32_t FISH_COLOR_2           = 0xFF42A5F5u; // bright blue
+constexpr uint32_t FISH_COLOR_3           = 0xFFE57373u; // coral pink
+constexpr uint32_t FISH_COLORS[FISH_COLOR_COUNT] = {
+    FISH_COLOR_0, FISH_COLOR_1, FISH_COLOR_2, FISH_COLOR_3,
+};
+constexpr uint32_t FISH_FIN_COLOR         = 0xFF222222u;
+constexpr uint64_t FISH_PRNG_SALT         = 0xF15F15F15F15F15Full;
+
+inline double ambient_clamp01(double value) noexcept {
+    if (value <= 0.0) return 0.0;
+    if (value >= 1.0) return 1.0;
+    return value;
+}
+
+inline double ambient_smoothstep01(double value) noexcept {
+    const double t = ambient_clamp01(value);
+    return t * t * (3.0 - 2.0 * t);
+}
+
+// §CPU: Winter draws a snow cap on every plain grass blade, which is the scene's
+// dominant render cost (~2,500 extra fills/frame). To lighten it, deterministically
+// cull a fixed fraction of plain blades (and their caps) in Winter only. The
+// decision is a pure hash of the blade's stable array index, so it is identical
+// across frames and across the Native/Win2D renderers, and survives cuts (the slot
+// model keeps indices stable). (hash & 3)==0 drops ~25% of blades.
+constexpr uint32_t WINTER_CULL_MASK = 3u;
+
+inline bool winter_blade_culled(uint32_t bladeIndex) noexcept {
+    uint32_t h = bladeIndex * 2654435761u;
+    h ^= h >> 13;
+    h *= 0x85ebca6bu;
+    h ^= h >> 16;
+    return (h & WINTER_CULL_MASK) == 0u;
+}
+
+inline double butterfly_wing_scale(double timeSeconds, double phaseY) noexcept {
+    const double raw = std::cos(timeSeconds * BUTTERFLY_FLUTTER_FREQ + phaseY);
+    if (raw < BUTTERFLY_FLUTTER_MIN_SCALE) return BUTTERFLY_FLUTTER_MIN_SCALE;
+    if (raw > 1.0) return 1.0;
+    return raw;
+}
+
+inline double bird_wing_scale(double timeSeconds, double wingPhaseOffset) noexcept {
+    const double t = 0.5 + 0.5 * std::cos(timeSeconds * BIRD_WING_FLAP_FREQ + wingPhaseOffset);
+    return BIRD_WING_FOLD_RATIO + (BIRD_WING_OPEN_RATIO - BIRD_WING_FOLD_RATIO) * t;
+}
+
+inline double bird_fade_alpha(double x, double vx, double monitorWidth) noexcept {
+    if (monitorWidth <= 0.0) return 0.0;
+    const double visibleSpan = monitorWidth;
+    const double fadeInDist = BIRD_FADE_IN_FRAC * visibleSpan;
+    const double fadeOutDist = BIRD_FADE_OUT_FRAC * visibleSpan;
+    double alpha = 1.0;
+    if (vx >= 0.0) {
+        if (fadeInDist > 0.0 && x < fadeInDist) alpha = std::min(alpha, ambient_clamp01((x + 50.0) / fadeInDist));
+        if (fadeOutDist > 0.0 && x > monitorWidth - fadeOutDist) alpha = std::min(alpha, ambient_clamp01((monitorWidth + 50.0 - x) / fadeOutDist));
+    } else {
+        if (fadeInDist > 0.0 && x > monitorWidth - fadeInDist) alpha = std::min(alpha, ambient_clamp01((monitorWidth + 50.0 - x) / fadeInDist));
+        if (fadeOutDist > 0.0 && x < fadeOutDist) alpha = std::min(alpha, ambient_clamp01((x + 50.0) / fadeOutDist));
+    }
+    return ambient_clamp01(alpha);
+}
+
+inline double firefly_blink_brightness(double timeSeconds, double blinkPeriod, double blinkPhase) noexcept {
+    if (blinkPeriod <= 0.0) return 0.0;
+    double cycleT = std::fmod(timeSeconds / blinkPeriod + blinkPhase, 1.0);
+    if (cycleT < 0.0) cycleT += 1.0;
+
+    if (cycleT >= FIREFLY_BLINK_DUTY) return 0.0;
+
+    const double fadeFrac = ambient_clamp01(FIREFLY_BLINK_FADE / blinkPeriod);
+    double brightness = 1.0;
+    if (fadeFrac > 0.0) {
+        if (cycleT < fadeFrac) {
+            brightness = ambient_smoothstep01(cycleT / fadeFrac);
+        } else if (cycleT > FIREFLY_BLINK_DUTY - fadeFrac) {
+            brightness = ambient_smoothstep01((FIREFLY_BLINK_DUTY - cycleT) / fadeFrac);
+        }
+    }
+    return ambient_clamp01(brightness);
+}
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/GrassWindow.cpp

@@ -0,0 +1,160 @@
+// GrassWindow.cpp
+
+#include "GrassWindow.h"
+
+#include <shellscalingapi.h>
+#pragma comment(lib, "Shcore.lib")
+#pragma comment(lib, "User32.lib")
+
+namespace desktopgrass {
+
+namespace {
+constexpr UINT_PTR kProp = 0; // placeholder; we use SetWindowLongPtr(GWLP_USERDATA)
+} // anonymous
+
+bool GrassWindow::RegisterWindowClass(HINSTANCE hInst) {
+    WNDCLASSEXW wc{};
+    wc.cbSize        = sizeof(wc);
+    wc.style         = CS_HREDRAW | CS_VREDRAW;
+    wc.lpfnWndProc   = GrassWindow::WndProc;
+    wc.hInstance     = hInst;
+    wc.lpszClassName = kWindowClassName;
+    wc.hCursor       = LoadCursorW(nullptr, IDC_ARROW);
+    wc.hbrBackground = nullptr; // we paint everything; never let GDI clear
+
+    ATOM atom = RegisterClassExW(&wc);
+    if (atom == 0) {
+        DWORD err = GetLastError();
+        if (err != ERROR_CLASS_ALREADY_EXISTS) {
+            return false;
+        }
+    }
+    return true;
+}
+
+GrassWindow::~GrassWindow() {
+    Destroy();
+}
+
+bool GrassWindow::Create(HINSTANCE hInst,
+                         const RECT& monitorBounds, UINT dpi,
+                         uint64_t seed, double density,
+                         double swaySpeed, double swayAmplitude)
+{
+    dpi_     = dpi == 0 ? 96 : dpi;
+    seed_    = seed;
+    density_ = density;
+    monitorBounds_ = monitorBounds;
+
+    // Compute window dims in pixels: full monitor width × (STRIP_HEIGHT +
+    // HEADROOM) DIP. Bottom-aligned to the monitor.
+    const int monitorW = monitorBounds.right  - monitorBounds.left;
+    const int heightPx = static_cast<int>(
+        ((STRIP_HEIGHT + HEADROOM) * dpi_ / 96.0) + 0.5);
+
+    screenBounds_.left   = monitorBounds.left;
+    screenBounds_.right  = monitorBounds.left + monitorW;
+    screenBounds_.bottom = monitorBounds.bottom;
+    screenBounds_.top    = monitorBounds.bottom - heightPx;
+
+    const DWORD exStyle =
+        WS_EX_LAYERED | WS_EX_TRANSPARENT | WS_EX_TOPMOST |
+        WS_EX_TOOLWINDOW | WS_EX_NOACTIVATE;
+    const DWORD style = WS_POPUP;
+
+    hwnd_ = CreateWindowExW(
+        exStyle, kWindowClassName, L"Desktop Grass",
+        style,
+        screenBounds_.left,  screenBounds_.top,
+        monitorW,            heightPx,
+        nullptr, nullptr, hInst, this);
+
+    if (!hwnd_) {
+        return false;
+    }
+
+    if (!renderer_.Initialize(hwnd_, monitorW, heightPx, dpi_, seed, density,
+                              swaySpeed, swayAmplitude)) {
+        DestroyWindow(hwnd_);
+        hwnd_ = nullptr;
+        return false;
+    }
+
+    renderer_.SetWindowOriginScreen(screenBounds_.left, screenBounds_.top);
+    return true;
+}
+
+void GrassWindow::Show() {
+    if (hwnd_) {
+        ShowWindow(hwnd_, SW_SHOWNOACTIVATE);
+    }
+}
+
+void GrassWindow::Destroy() {
+    if (hwnd_) {
+        DestroyWindow(hwnd_);
+        hwnd_ = nullptr;
+    }
+}
+
+void GrassWindow::RenderFrame(double dt,
+                              const InputEvent* events, std::size_t numEvents)
+{
+    renderer_.RenderFrame(dt, events, numEvents);
+}
+
+LRESULT CALLBACK GrassWindow::WndProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp) {
+    GrassWindow* self = nullptr;
+    if (msg == WM_NCCREATE) {
+        auto* cs = reinterpret_cast<CREATESTRUCTW*>(lp);
+        self = reinterpret_cast<GrassWindow*>(cs->lpCreateParams);
+        SetWindowLongPtrW(hwnd, GWLP_USERDATA, reinterpret_cast<LONG_PTR>(self));
+        if (self) self->hwnd_ = hwnd;
+    } else {
+        self = reinterpret_cast<GrassWindow*>(GetWindowLongPtrW(hwnd, GWLP_USERDATA));
+    }
+    if (self) return self->HandleMessage(msg, wp, lp);
+    return DefWindowProcW(hwnd, msg, wp, lp);
+}
+
+LRESULT GrassWindow::HandleMessage(UINT msg, WPARAM wp, LPARAM lp) {
+    switch (msg) {
+        case WM_CLOSE:
+            // The smoke harness sends WM_CLOSE. Forward to the main thread as
+            // a request to terminate the message loop.
+            PostQuitMessage(0);
+            return 0;
+
+        case WM_DPICHANGED: {
+            const UINT newDpi = HIWORD(wp);
+            auto* rect = reinterpret_cast<const RECT*>(lp);
+            if (rect) {
+                SetWindowPos(hwnd_, nullptr,
+                             rect->left, rect->top,
+                             rect->right - rect->left,
+                             rect->bottom - rect->top,
+                             SWP_NOZORDER | SWP_NOACTIVATE);
+                renderer_.Resize(rect->right - rect->left,
+                                 rect->bottom - rect->top, newDpi);
+                dpi_ = newDpi;
+                screenBounds_ = *rect;
+                renderer_.SetWindowOriginScreen(rect->left, rect->top);
+                // Mirror the Win2D rebuild: regenerate the blade layout for the
+                // new DIP width using the same per-monitor seed so the result is
+                // identical to a fresh launch at this DPI. Reuses the stored
+                // seed_/density_; sway scales and scene/critter/cut state are
+                // preserved inside RegenerateForDpi.
+                renderer_.RegenerateForDpi(seed_, density_);
+            }
+            return 0;
+        }
+
+        case WM_DESTROY:
+            return 0;
+
+        default:
+            return DefWindowProcW(hwnd_, msg, wp, lp);
+    }
+}
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/GrassWindow.h

@@ -0,0 +1,61 @@
+// GrassWindow.h
+//
+// One HWND + one Renderer per monitor. Layered, click-through, topmost,
+// no-activate, tool-window — see WS_EX flags listed in the plan and asserted
+// by tests/smoke.
+
+#pragma once
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+#include <memory>
+
+#include "Renderer.h"
+#include "MouseHook.h"
+
+namespace desktopgrass {
+
+class GrassWindow {
+public:
+    static constexpr const wchar_t* kWindowClassName = L"DesktopGrass.Native.Window";
+    static constexpr UINT           kWmAppQuit       = WM_APP + 1;
+
+    static bool RegisterWindowClass(HINSTANCE hInst);
+
+    GrassWindow() = default;
+    ~GrassWindow();
+
+    GrassWindow(const GrassWindow&)            = delete;
+    GrassWindow& operator=(const GrassWindow&) = delete;
+
+    // Creates the HWND, attaches a Renderer, generates blades using `seed`.
+    bool Create(HINSTANCE hInst,
+                const RECT& monitorBounds, UINT dpi,
+                uint64_t seed, double density,
+                double swaySpeed = 1.0, double swayAmplitude = 1.0);
+
+    void Show();
+    void Destroy();
+    void RenderFrame(double dt,
+                     const InputEvent* events, std::size_t numEvents);
+
+    HWND      GetHwnd()  const { return hwnd_; }
+    Renderer& GetRenderer()     { return renderer_; }
+    const RECT& GetScreenBounds() const { return screenBounds_; }
+    const RECT& GetMonitorBounds() const { return monitorBounds_; }
+
+private:
+    static LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp);
+    LRESULT HandleMessage(UINT msg, WPARAM wp, LPARAM lp);
+
+    HWND       hwnd_ = nullptr;
+    Renderer   renderer_;
+    RECT       screenBounds_{}; // window screen-rect (left, top, right, bottom)
+    RECT       monitorBounds_{}; // monitor work-area rect used for persistence keys
+    UINT       dpi_   = 96;
+    uint64_t   seed_  = 0;
+    double     density_ = 1.0;
+};
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/Json.h

@@ -0,0 +1,355 @@
+// Json.h
+//
+// Minimal, dependency-free JSON reader shared by the persistence and config
+// loaders. Tolerates JSONC niceties — // line comments, /* block */ comments,
+// and trailing commas — so the human-editable config.json can be annotated.
+// Header-only: every free helper is marked inline to stay ODR-safe across the
+// translation units that include it.
+
+#pragma once
+
+#include <cctype>
+#include <cstdio>
+#include <cstdlib>
+#include <map>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <vector>
+
+namespace desktopgrass::json {
+
+// ASCII-only lowercase fold (a-z), locale-independent, so object member keys
+// can be matched case-insensitively to mirror the Win2D loader's
+// PropertyNameCaseInsensitive=true / OrdinalIgnoreCase behavior. Non-ASCII and
+// non-letter bytes pass through unchanged.
+inline std::string AsciiLower(std::string_view text) {
+    std::string out(text);
+    for (char& c : out) {
+        if (c >= 'A' && c <= 'Z') {
+            c = static_cast<char>(c - 'A' + 'a');
+        }
+    }
+    return out;
+}
+
+struct Value {
+    enum class Type { Null, Bool, Number, String, Array, Object };
+
+    Type type = Type::Null;
+    bool boolValue = false;
+    double numberValue = 0.0;
+    std::string stringValue;
+    std::vector<Value> arrayValue;
+    std::map<std::string, Value> objectValue;
+};
+
+class Parser {
+public:
+    explicit Parser(std::string_view text) : text_(text) {}
+
+    bool Parse(Value& out) {
+        SkipWhitespace();
+        if (!ParseValue(out)) {
+            return false;
+        }
+        SkipWhitespace();
+        return pos_ == text_.size();
+    }
+
+private:
+    void SkipWhitespace() noexcept {
+        while (pos_ < text_.size()) {
+            const unsigned char c = static_cast<unsigned char>(text_[pos_]);
+            if (std::isspace(c)) {
+                ++pos_;
+                continue;
+            }
+            // JSONC: skip // line and /* block */ comments.
+            if (c == '/' && pos_ + 1 < text_.size()) {
+                if (text_[pos_ + 1] == '/') {
+                    pos_ += 2;
+                    while (pos_ < text_.size() && text_[pos_] != '\n') ++pos_;
+                    continue;
+                }
+                if (text_[pos_ + 1] == '*') {
+                    pos_ += 2;
+                    while (pos_ + 1 < text_.size() &&
+                           !(text_[pos_] == '*' && text_[pos_ + 1] == '/')) {
+                        ++pos_;
+                    }
+                    pos_ = (pos_ + 1 < text_.size()) ? pos_ + 2 : text_.size();
+                    continue;
+                }
+            }
+            break;
+        }
+    }
+
+    bool Match(std::string_view literal) noexcept {
+        if (text_.substr(pos_, literal.size()) != literal) {
+            return false;
+        }
+        pos_ += literal.size();
+        return true;
+    }
+
+    bool ParseValue(Value& out) {
+        SkipWhitespace();
+        if (pos_ >= text_.size()) return false;
+
+        const char c = text_[pos_];
+        if (c == '{') return ParseObject(out);
+        if (c == '[') return ParseArray(out);
+        if (c == '"') {
+            out.type = Value::Type::String;
+            return ParseString(out.stringValue);
+        }
+        if (c == 't') {
+            if (!Match("true")) return false;
+            out.type = Value::Type::Bool;
+            out.boolValue = true;
+            return true;
+        }
+        if (c == 'f') {
+            if (!Match("false")) return false;
+            out.type = Value::Type::Bool;
+            out.boolValue = false;
+            return true;
+        }
+        if (c == 'n') {
+            if (!Match("null")) return false;
+            out.type = Value::Type::Null;
+            return true;
+        }
+        if (c == '-' || (c >= '0' && c <= '9')) {
+            return ParseNumber(out);
+        }
+        return false;
+    }
+
+    bool ParseObject(Value& out) {
+        if (text_[pos_] != '{') return false;
+        ++pos_;
+        out.type = Value::Type::Object;
+        out.objectValue.clear();
+
+        SkipWhitespace();
+        if (pos_ < text_.size() && text_[pos_] == '}') {
+            ++pos_;
+            return true;
+        }
+
+        while (pos_ < text_.size()) {
+            SkipWhitespace();
+            std::string key;
+            if (!ParseString(key)) return false;
+            SkipWhitespace();
+            if (pos_ >= text_.size() || text_[pos_] != ':') return false;
+            ++pos_;
+
+            Value value;
+            if (!ParseValue(value)) return false;
+            // Normalize keys to ASCII-lowercase so config/state lookups are
+            // case-insensitive (matching the Win2D loader). Monitor keys like
+            // "1920x1080@0,0" contain no uppercase letters, so this is a no-op
+            // for them.
+            out.objectValue.emplace(AsciiLower(key), std::move(value));
+
+            SkipWhitespace();
+            if (pos_ >= text_.size()) return false;
+            if (text_[pos_] == '}') {
+                ++pos_;
+                return true;
+            }
+            if (text_[pos_] != ',') return false;
+            ++pos_;
+            // JSONC: allow a trailing comma before the closing brace.
+            SkipWhitespace();
+            if (pos_ < text_.size() && text_[pos_] == '}') {
+                ++pos_;
+                return true;
+            }
+        }
+        return false;
+    }
+
+    bool ParseArray(Value& out) {
+        if (text_[pos_] != '[') return false;
+        ++pos_;
+        out.type = Value::Type::Array;
+        out.arrayValue.clear();
+
+        SkipWhitespace();
+        if (pos_ < text_.size() && text_[pos_] == ']') {
+            ++pos_;
+            return true;
+        }
+
+        while (pos_ < text_.size()) {
+            Value value;
+            if (!ParseValue(value)) return false;
+            out.arrayValue.push_back(std::move(value));
+
+            SkipWhitespace();
+            if (pos_ >= text_.size()) return false;
+            if (text_[pos_] == ']') {
+                ++pos_;
+                return true;
+            }
+            if (text_[pos_] != ',') return false;
+            ++pos_;
+            // JSONC: allow a trailing comma before the closing bracket.
+            SkipWhitespace();
+            if (pos_ < text_.size() && text_[pos_] == ']') {
+                ++pos_;
+                return true;
+            }
+        }
+        return false;
+    }
+
+    bool ParseString(std::string& out) {
+        if (pos_ >= text_.size() || text_[pos_] != '"') return false;
+        ++pos_;
+        out.clear();
+
+        while (pos_ < text_.size()) {
+            const char c = text_[pos_++];
+            if (c == '"') return true;
+            if (c == '\\') {
+                if (pos_ >= text_.size()) return false;
+                const char esc = text_[pos_++];
+                switch (esc) {
+                case '"': out.push_back('"'); break;
+                case '\\': out.push_back('\\'); break;
+                case '/': out.push_back('/'); break;
+                case 'b': out.push_back('\b'); break;
+                case 'f': out.push_back('\f'); break;
+                case 'n': out.push_back('\n'); break;
+                case 'r': out.push_back('\r'); break;
+                case 't': out.push_back('\t'); break;
+                case 'u':
+                    if (pos_ + 4 > text_.size()) return false;
+                    out.push_back('?');
+                    pos_ += 4;
+                    break;
+                default:
+                    return false;
+                }
+            } else {
+                out.push_back(c);
+            }
+        }
+        return false;
+    }
+
+    bool ParseNumber(Value& out) {
+        const std::size_t start = pos_;
+        if (text_[pos_] == '-') ++pos_;
+        if (pos_ >= text_.size()) return false;
+
+        if (text_[pos_] == '0') {
+            ++pos_;
+        } else if (text_[pos_] >= '1' && text_[pos_] <= '9') {
+            while (pos_ < text_.size() && text_[pos_] >= '0' && text_[pos_] <= '9') {
+                ++pos_;
+            }
+        } else {
+            return false;
+        }
+
+        if (pos_ < text_.size() && text_[pos_] == '.') {
+            ++pos_;
+            if (pos_ >= text_.size() || text_[pos_] < '0' || text_[pos_] > '9') return false;
+            while (pos_ < text_.size() && text_[pos_] >= '0' && text_[pos_] <= '9') {
+                ++pos_;
+            }
+        }
+
+        if (pos_ < text_.size() && (text_[pos_] == 'e' || text_[pos_] == 'E')) {
+            ++pos_;
+            if (pos_ < text_.size() && (text_[pos_] == '+' || text_[pos_] == '-')) ++pos_;
+            if (pos_ >= text_.size() || text_[pos_] < '0' || text_[pos_] > '9') return false;
+            while (pos_ < text_.size() && text_[pos_] >= '0' && text_[pos_] <= '9') {
+                ++pos_;
+            }
+        }
+
+        const std::string token(text_.substr(start, pos_ - start));
+        char* endPtr = nullptr;
+        const double value = std::strtod(token.c_str(), &endPtr);
+        if (endPtr == token.c_str() || *endPtr != '\0') return false;
+
+        out.type = Value::Type::Number;
+        out.numberValue = value;
+        return true;
+    }
+
+    std::string_view text_;
+    std::size_t pos_ = 0;
+};
+
+inline bool Parse(std::string_view text, Value& out) {
+    Parser parser(text);
+    return parser.Parse(out);
+}
+
+inline const Value* FindMember(const Value& object, const std::string& name) {
+    if (object.type != Value::Type::Object) return nullptr;
+    // Keys are stored ASCII-lowercased at parse time, so fold the lookup name
+    // the same way for case-insensitive matching.
+    const auto it = object.objectValue.find(AsciiLower(name));
+    return it == object.objectValue.end() ? nullptr : &it->second;
+}
+
+inline std::optional<int> ReadInt(const Value& object, const std::string& name) {
+    const Value* value = FindMember(object, name);
+    if (!value || value->type != Value::Type::Number) return std::nullopt;
+    return static_cast<int>(value->numberValue);
+}
+
+inline std::optional<double> ReadDouble(const Value& object, const std::string& name) {
+    const Value* value = FindMember(object, name);
+    if (!value || value->type != Value::Type::Number) return std::nullopt;
+    return value->numberValue;
+}
+
+inline std::optional<bool> ReadBool(const Value& object, const std::string& name) {
+    const Value* value = FindMember(object, name);
+    if (!value || value->type != Value::Type::Bool) return std::nullopt;
+    return value->boolValue;
+}
+
+inline std::optional<std::string> ReadString(const Value& object, const std::string& name) {
+    const Value* value = FindMember(object, name);
+    if (!value || value->type != Value::Type::String) return std::nullopt;
+    return value->stringValue;
+}
+
+inline std::string Escape(std::string_view text) {
+    std::string out;
+    for (char c : text) {
+        switch (c) {
+        case '"': out += "\\\""; break;
+        case '\\': out += "\\\\"; break;
+        case '\b': out += "\\b"; break;
+        case '\f': out += "\\f"; break;
+        case '\n': out += "\\n"; break;
+        case '\r': out += "\\r"; break;
+        case '\t': out += "\\t"; break;
+        default:
+            if (static_cast<unsigned char>(c) < 0x20) {
+                char buffer[7]{};
+                std::snprintf(buffer, sizeof(buffer), "\\u%04x", static_cast<unsigned char>(c));
+                out += buffer;
+            } else {
+                out.push_back(c);
+            }
+            break;
+        }
+    }
+    return out;
+}
+
+} // namespace desktopgrass::json

src/modules/DesktopGrass/DesktopGrass.Native/src/MouseHook.cpp

@@ -0,0 +1,72 @@
+// MouseHook.cpp
+
+#include "MouseHook.h"
+
+#include <atomic>
+#include <chrono>
+
+namespace desktopgrass {
+
+namespace {
+
+std::atomic<MouseEventQueue*> g_queue{nullptr};
+HHOOK                         g_hook = nullptr;
+LARGE_INTEGER                 g_qpcFreq{};
+LARGE_INTEGER                 g_qpcStart{};
+
+double now_seconds() noexcept {
+    LARGE_INTEGER c;
+    QueryPerformanceCounter(&c);
+    return static_cast<double>(c.QuadPart - g_qpcStart.QuadPart) /
+           static_cast<double>(g_qpcFreq.QuadPart);
+}
+
+LRESULT CALLBACK LowLevelMouseProc(int nCode, WPARAM wParam, LPARAM lParam) {
+    // Per spec: always pass the event through. Never consume.
+    if (nCode == HC_ACTION) {
+        MouseEventQueue* q = g_queue.load(std::memory_order_acquire);
+        if (q) {
+            const MSLLHOOKSTRUCT* m = reinterpret_cast<const MSLLHOOKSTRUCT*>(lParam);
+            RawMouseEvent ev{};
+            ev.timeSeconds = now_seconds();
+            ev.screenX     = m->pt.x;
+            ev.screenY     = m->pt.y;
+
+            switch (wParam) {
+                case WM_MOUSEMOVE:
+                    ev.type = EventType::Move;
+                    q->push(ev);
+                    break;
+                case WM_LBUTTONDOWN:
+                    ev.type = EventType::Click;
+                    q->push(ev);
+                    break;
+                default:
+                    break;
+            }
+        }
+    }
+    return CallNextHookEx(nullptr, nCode, wParam, lParam);
+}
+
+} // anonymous
+
+bool install_mouse_hook(MouseEventQueue* queue) noexcept {
+    if (g_hook) return false;
+    QueryPerformanceFrequency(&g_qpcFreq);
+    QueryPerformanceCounter(&g_qpcStart);
+    g_queue.store(queue, std::memory_order_release);
+    g_hook = SetWindowsHookExW(WH_MOUSE_LL, LowLevelMouseProc,
+                               GetModuleHandleW(nullptr), 0);
+    return g_hook != nullptr;
+}
+
+void uninstall_mouse_hook() noexcept {
+    if (g_hook) {
+        UnhookWindowsHookEx(g_hook);
+        g_hook = nullptr;
+    }
+    g_queue.store(nullptr, std::memory_order_release);
+}
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/MouseHook.h

@@ -0,0 +1,71 @@
+// MouseHook.h
+//
+// WH_MOUSE_LL global low-level mouse hook. The callback runs on Windows'
+// dedicated hook thread and must return very quickly (≤ ~200 µs is the kind of
+// budget where Windows un-installs you if you exceed it). It pushes a fixed-size
+// snapshot of the event into a lock-free single-producer / single-consumer ring
+// buffer. The render loop drains the queue once per frame.
+
+#pragma once
+
+#include <atomic>
+#include <cstddef>
+#include <cstdint>
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+#include "Sim.h"
+
+namespace desktopgrass {
+
+struct RawMouseEvent {
+    EventType type;
+    double    timeSeconds;
+    int32_t   screenX; // virtual screen coords, raw from the hook
+    int32_t   screenY;
+};
+
+class MouseEventQueue {
+public:
+    static constexpr std::size_t CAPACITY = 1024; // power of two
+
+    MouseEventQueue() : head_(0), tail_(0) {}
+
+    // Producer side (low-level hook thread). Returns false if full (we drop the
+    // event rather than block — UI freezes are worse than a missed gust).
+    bool push(const RawMouseEvent& e) noexcept {
+        const std::size_t head = head_.load(std::memory_order_relaxed);
+        const std::size_t next = (head + 1) & (CAPACITY - 1);
+        if (next == tail_.load(std::memory_order_acquire)) {
+            return false; // full
+        }
+        buffer_[head] = e;
+        head_.store(next, std::memory_order_release);
+        return true;
+    }
+
+    // Consumer side (render thread). Returns the number of events read.
+    std::size_t drain(RawMouseEvent* dst, std::size_t maxCount) noexcept {
+        std::size_t n = 0;
+        std::size_t tail = tail_.load(std::memory_order_relaxed);
+        const std::size_t head = head_.load(std::memory_order_acquire);
+        while (tail != head && n < maxCount) {
+            dst[n++] = buffer_[tail];
+            tail = (tail + 1) & (CAPACITY - 1);
+        }
+        tail_.store(tail, std::memory_order_release);
+        return n;
+    }
+
+private:
+    RawMouseEvent            buffer_[CAPACITY];
+    std::atomic<std::size_t> head_; // producer
+    std::atomic<std::size_t> tail_; // consumer
+};
+
+// Singleton-style install / uninstall. Only one hook per process.
+bool        install_mouse_hook(MouseEventQueue* queue) noexcept;
+void        uninstall_mouse_hook() noexcept;
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/Pacing.cpp

@@ -0,0 +1,60 @@
+// Pacing.cpp
+
+#include "Pacing.h"
+
+namespace desktopgrass {
+
+namespace {
+
+// SetWaitableTimer's lpDueTime takes 100-ns intervals. Negative values mean
+// "relative to now". One second == 10,000,000 hundred-ns units.
+constexpr double kHundredNsPerSec = 10'000'000.0;
+
+} // anonymous
+
+FramePacer::FramePacer() {
+    // CREATE_WAITABLE_TIMER_HIGH_RESOLUTION (0x00000002) requires Windows 10
+    // 1803+. DesktopGrass already requires Windows 10 1809+ (see README), so
+    // creation should succeed in supported environments. The nullptr returned
+    // on any older system is fine: WaitUntilNextFrame falls back to the
+    // legacy MWFMOe(NULL, waitMs, ...) path so behaviour degrades gracefully.
+    timer_ = CreateWaitableTimerExW(
+        nullptr, nullptr,
+        CREATE_WAITABLE_TIMER_HIGH_RESOLUTION,
+        TIMER_ALL_ACCESS);
+}
+
+FramePacer::~FramePacer() {
+    if (timer_) {
+        CloseHandle(timer_);
+        timer_ = nullptr;
+    }
+}
+
+void FramePacer::WaitUntilNextFrame(double waitSec) {
+    if (waitSec <= 0.0) return;
+
+    if (timer_) {
+        // Relative due time in 100-ns units; round down so we never sleep
+        // longer than asked. SetWaitableTimer will fire immediately if the
+        // computed magnitude is zero.
+        LARGE_INTEGER due{};
+        const double hundredNs = waitSec * kHundredNsPerSec;
+        due.QuadPart = -static_cast<LONGLONG>(hundredNs);
+
+        if (SetWaitableTimer(timer_, &due, 0, nullptr, nullptr, FALSE)) {
+            MsgWaitForMultipleObjectsEx(
+                1, &timer_, INFINITE, QS_ALLINPUT, MWMO_INPUTAVAILABLE);
+            return;
+        }
+        // SetWaitableTimer can theoretically fail (e.g. handle revoked);
+        // fall through to the legacy wait so the loop still makes progress.
+    }
+
+    // Legacy ms-resolution wait. Round to nearest millisecond.
+    const DWORD waitMs = static_cast<DWORD>(waitSec * 1000.0 + 0.5);
+    MsgWaitForMultipleObjectsEx(
+        0, nullptr, waitMs, QS_ALLINPUT, MWMO_INPUTAVAILABLE);
+}
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/Pacing.h

@@ -0,0 +1,48 @@
+// Pacing.h
+//
+// Frame pacing helper.
+//
+// The default Windows system timer resolution is ~15.6 ms, which clamps any
+// `MsgWaitForMultipleObjectsEx(NULL, waitMs, ...)`-based loop to roughly
+// 64 fps even when the caller asks for a shorter wait. At our 30 fps target
+// that's actually below the requested cadence: a frame that asks for ~30 ms
+// of wait ends up paying for two ~15.6 ms ticks (~31 ms) on the lucky path
+// and three ticks (~46 ms) on the unlucky one, producing visibly uneven
+// motion and dt_p95 around 48 ms.
+//
+// `FramePacer` uses a per-process high-resolution waitable timer
+// (CREATE_WAITABLE_TIMER_HIGH_RESOLUTION, Windows 10 1803+) so the wait
+// honours its argument to roughly sub-ms granularity without changing the
+// system-wide timer resolution. If the high-res timer cannot be created the
+// pacer transparently falls back to the legacy ms-resolution wait, matching
+// the pre-fix behaviour.
+
+#pragma once
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+namespace desktopgrass {
+
+class FramePacer {
+public:
+    FramePacer();
+    ~FramePacer();
+
+    FramePacer(const FramePacer&)            = delete;
+    FramePacer& operator=(const FramePacer&) = delete;
+
+    // True iff the high-resolution waitable timer was created. False means the
+    // pacer is operating in legacy MWFMOe(NULL, waitMs, ...) mode.
+    bool IsHighResolution() const { return timer_ != nullptr; }
+
+    // Block until `waitSec` elapses or input arrives in the calling thread's
+    // message queue (QS_ALLINPUT, MWMO_INPUTAVAILABLE). Returns immediately
+    // when `waitSec <= 0`.
+    void WaitUntilNextFrame(double waitSec);
+
+private:
+    HANDLE timer_ = nullptr;
+};
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/Persistence.cpp

@@ -0,0 +1,283 @@
+#include "Persistence.h"
+
+#include "Json.h"
+
+#include <Windows.h>
+
+#include <algorithm>
+#include <chrono>
+#include <cctype>
+#include <cstdio>
+#include <cstdlib>
+#include <ctime>
+#include <cmath>
+#include <filesystem>
+#include <fstream>
+#include <iomanip>
+#include <map>
+#include <optional>
+#include <sstream>
+#include <string_view>
+#include <utility>
+
+namespace desktopgrass::persistence {
+namespace {
+
+using desktopgrass::json::FindMember;
+using desktopgrass::json::ReadBool;
+using desktopgrass::json::ReadDouble;
+using desktopgrass::json::ReadInt;
+using desktopgrass::json::ReadString;
+using JsonValue = desktopgrass::json::Value;
+using JsonParser = desktopgrass::json::Parser;
+
+std::optional<std::wstring> g_stateFilePathForTest;
+constexpr int kCurrentVersion = 2;
+
+std::string JsonEscape(std::string_view text) {
+    return desktopgrass::json::Escape(text);
+}
+
+std::string SceneToString(Scene scene) noexcept {
+    switch (scene) {
+    case Scene::Grass:  return "Grass";
+    case Scene::Desert: return "Desert";
+    case Scene::Winter: return "Winter";
+    case Scene::Autumn: return "Autumn";
+    case Scene::Ocean:  return "Ocean";
+    }
+    return "Grass";
+}
+
+Scene SceneFromString(const std::string& scene) noexcept {
+    if (scene == "Desert") return Scene::Desert;
+    if (scene == "Winter") return Scene::Winter;
+    if (scene == "Autumn") return Scene::Autumn;
+    if (scene == "Ocean") return Scene::Ocean;
+    return Scene::Grass;
+}
+
+std::string CritterToString(CritterKind critter) noexcept {
+    switch (critter) {
+    case CritterKind::None:  return "None";
+    case CritterKind::Sheep: return "Sheep";
+    case CritterKind::Cat:   return "Cat";
+    case CritterKind::Bunny: return "Bunny";
+    }
+    return "None";
+}
+
+CritterKind CritterFromString(const std::string& critter) noexcept {
+    if (critter == "Sheep") return CritterKind::Sheep;
+    if (critter == "Cat") return CritterKind::Cat;
+    if (critter == "Bunny") return CritterKind::Bunny;
+    return CritterKind::None;
+}
+
+std::string CurrentUtcTimestamp() {
+    const auto now = std::chrono::system_clock::now();
+    const std::time_t time = std::chrono::system_clock::to_time_t(now);
+    std::tm utc{};
+    gmtime_s(&utc, &time);
+
+    std::ostringstream out;
+    out << std::put_time(&utc, "%Y-%m-%dT%H:%M:%SZ");
+    return out.str();
+}
+
+bool TryParseMonitorKey(const std::string& key, MonitorState& monitor) {
+    int consumed = 0;
+    const int matched = sscanf_s(key.c_str(), "%dx%d@%d,%d%n",
+                                    &monitor.width,
+                                    &monitor.height,
+                                    &monitor.left,
+                                    &monitor.top,
+                                    &consumed);
+    return matched == 4 && consumed == static_cast<int>(key.size());
+}
+
+std::string Serialize(const AppState& state) {
+    std::ostringstream out;
+    out << std::setprecision(17);
+    out << "{\n";
+    out << "  \"version\": " << kCurrentVersion << ",\n";
+    out << "  \"savedAt\": \"" << CurrentUtcTimestamp() << "\",\n";
+    out << "  \"scene\": \"" << SceneToString(state.scene) << "\",\n";
+    out << "  \"critter\": \"" << CritterToString(state.critter) << "\",\n";
+    out << "  \"critterCount\": " << state.critterCountOverride << ",\n";
+    out << "  \"autoStart\": " << (state.autoStart ? "true" : "false") << ",\n";
+    out << "  \"monitors\": {\n";
+
+    for (std::size_t i = 0; i < state.monitors.size(); ++i) {
+        const MonitorState& monitor = state.monitors[i];
+        out << "    \"" << JsonEscape(MonitorKey(monitor)) << "\": {\n";
+        out << "      \"cuts\": [";
+        if (!monitor.cuts.empty()) {
+            out << "\n";
+            for (std::size_t j = 0; j < monitor.cuts.size(); ++j) {
+                const CutRecord& cut = monitor.cuts[j];
+                out << "        { \"bladeIndex\": " << cut.bladeIndex
+                    << ", \"cutTime\": " << cut.cutTime << " }";
+                if (j + 1 < monitor.cuts.size()) out << ",";
+                out << "\n";
+            }
+            out << "      ";
+        }
+        out << "]\n";
+        out << "    }";
+        if (i + 1 < state.monitors.size()) out << ",";
+        out << "\n";
+    }
+
+    out << "  }\n";
+    out << "}\n";
+    return out.str();
+}
+
+bool ParseAppState(const JsonValue& root, AppState& out) {
+    if (root.type != JsonValue::Type::Object) return false;
+
+    const int version = ReadInt(root, "version").value_or(0);
+    if (version != 1 && version != kCurrentVersion) {
+        OutputDebugStringA("DesktopGrass persistence: unsupported state.json version; starting fresh.\n");
+        return false;
+    }
+
+    AppState parsed;
+    parsed.version = kCurrentVersion;
+
+    auto sceneName = ReadString(root, "scene");
+    if (!sceneName) sceneName = ReadString(root, "currentScene");
+    parsed.scene = SceneFromString(sceneName.value_or("Grass"));
+
+    auto critterName = ReadString(root, "critter");
+    if (!critterName) critterName = ReadString(root, "currentCritter");
+    parsed.critter = CritterFromString(critterName.value_or("None"));
+
+    auto critterCount = ReadInt(root, "critterCount");
+    if (!critterCount) critterCount = ReadInt(root, "critterCountOverride");
+    parsed.critterCountOverride = critterCount.value_or(0);
+    if (parsed.critterCountOverride < 0 || parsed.critterCountOverride > PET_COUNT_MAX_PER_MONITOR) {
+        parsed.critterCountOverride = 0;
+    }
+    parsed.autoStart = ReadBool(root, "autoStart").value_or(false);
+
+    const JsonValue* monitors = FindMember(root, "monitors");
+    if (monitors && monitors->type == JsonValue::Type::Object) {
+        for (const auto& [key, value] : monitors->objectValue) {
+            MonitorState monitor;
+            if (!TryParseMonitorKey(key, monitor)) {
+                continue;
+            }
+
+            const JsonValue* cuts = FindMember(value, "cuts");
+            if (cuts && cuts->type == JsonValue::Type::Array) {
+                for (const JsonValue& cutValue : cuts->arrayValue) {
+                    if (cutValue.type != JsonValue::Type::Object) continue;
+                    const auto bladeIndex = ReadInt(cutValue, "bladeIndex");
+                    const auto cutTime = ReadDouble(cutValue, "cutTime");
+                    if (!bladeIndex || !cutTime) continue;
+                    monitor.cuts.push_back(CutRecord{ *bladeIndex, *cutTime });
+                }
+            }
+            parsed.monitors.push_back(std::move(monitor));
+        }
+    }
+
+    out = std::move(parsed);
+    return true;
+}
+
+std::wstring DefaultStateFilePath() {
+    wchar_t* localAppData = nullptr;
+    std::size_t length = 0;
+    _wdupenv_s(&localAppData, &length, L"LOCALAPPDATA");
+
+    std::filesystem::path path = localAppData && length > 0
+        ? std::filesystem::path(localAppData)
+        : std::filesystem::current_path();
+
+    if (localAppData) {
+        std::free(localAppData);
+    }
+
+    path /= L"DesktopGrass";
+    path /= L"state.json";
+    return path.wstring();
+}
+
+} // namespace
+
+std::string MonitorKey(int width, int height, int left, int top) {
+    return std::to_string(width) + "x" + std::to_string(height) + "@"
+         + std::to_string(left) + "," + std::to_string(top);
+}
+
+std::string MonitorKey(const MonitorState& monitor) {
+    return MonitorKey(monitor.width, monitor.height, monitor.left, monitor.top);
+}
+
+std::wstring GetStateFilePath() {
+    if (g_stateFilePathForTest) {
+        return *g_stateFilePathForTest;
+    }
+    return DefaultStateFilePath();
+}
+
+void SetStateFilePathForTest(const std::wstring& path) {
+    if (path.empty()) {
+        g_stateFilePathForTest.reset();
+    } else {
+        g_stateFilePathForTest = path;
+    }
+}
+
+bool LoadAppState(AppState& out) {
+    const std::filesystem::path path(GetStateFilePath());
+    std::ifstream file(path, std::ios::binary);
+    if (!file) {
+        return false;
+    }
+
+    std::ostringstream buffer;
+    buffer << file.rdbuf();
+
+    const std::string json = buffer.str();
+    JsonValue root;
+    JsonParser parser(json);
+    if (!parser.Parse(root)) {
+        OutputDebugStringA("DesktopGrass persistence: malformed state.json; starting fresh.\n");
+        return false;
+    }
+
+    return ParseAppState(root, out);
+}
+
+bool SaveAppState(const AppState& state) {
+    const std::filesystem::path path(GetStateFilePath());
+    const std::filesystem::path directory = path.parent_path();
+    if (!directory.empty()) {
+        std::error_code ec;
+        std::filesystem::create_directories(directory, ec);
+        if (ec) return false;
+    }
+
+    const std::filesystem::path tempPath(path.wstring() + L".tmp");
+    {
+        std::ofstream file(tempPath, std::ios::binary | std::ios::trunc);
+        if (!file) return false;
+        const std::string json = Serialize(state);
+        file.write(json.data(), static_cast<std::streamsize>(json.size()));
+        if (!file) return false;
+    }
+
+    if (!MoveFileExW(tempPath.c_str(), path.c_str(), MOVEFILE_REPLACE_EXISTING | MOVEFILE_WRITE_THROUGH)) {
+        std::error_code ec;
+        std::filesystem::remove(tempPath, ec);
+        return false;
+    }
+
+    return true;
+}
+
+} // namespace desktopgrass::persistence

src/modules/DesktopGrass/DesktopGrass.Native/src/Persistence.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include "Constants.h"
+
+#include <string>
+#include <vector>
+
+namespace desktopgrass::persistence {
+
+struct CutRecord {
+    int bladeIndex = 0;
+    double cutTime = 0.0;
+};
+
+struct MonitorState {
+    int width = 0;
+    int height = 0;
+    int left = 0;
+    int top = 0;
+    std::vector<CutRecord> cuts;
+};
+
+struct AppState {
+    int version = 2;
+    Scene scene = Scene::Grass;
+    CritterKind critter = CritterKind::None;
+    int critterCountOverride = 0;
+    bool autoStart = false;
+    std::vector<MonitorState> monitors;
+};
+
+bool LoadAppState(AppState& out);
+bool SaveAppState(const AppState& state);
+std::wstring GetStateFilePath();
+void SetStateFilePathForTest(const std::wstring& path);
+std::string MonitorKey(int width, int height, int left, int top);
+std::string MonitorKey(const MonitorState& monitor);
+
+} // namespace desktopgrass::persistence

src/modules/DesktopGrass/DesktopGrass.Native/src/Renderer.h

@@ -0,0 +1,174 @@
+// Renderer.h
+//
+// Per-window Direct2D + DXGI renderer attached to a DirectComposition target.
+// Owns the swap chain, the D2D device context bound to it, and the per-window
+// Sim. Renders the procedural grass once per frame.
+
+#pragma once
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+#include <wrl/client.h>
+#include <d3d11.h>
+#include <dxgi1_3.h>
+#include <d2d1_3.h>
+#include <dcomp.h>
+#include <dwrite.h>
+
+#include <unordered_map>
+
+#include "Sim.h"
+
+namespace desktopgrass {
+
+class Renderer {
+public:
+    Renderer() = default;
+    ~Renderer();
+
+    // Sets up D3D / D2D / DComp on `hwnd` of the given width × height in DIPs,
+    // and generates the initial blade list with `seed`. Returns false on
+    // failure (logged via OutputDebugString).
+    bool Initialize(HWND hwnd, int widthPx, int heightPx,
+                    UINT dpi, uint64_t seed, double density,
+                    double swaySpeed = 1.0, double swayAmplitude = 1.0);
+
+    // Resize the swap chain & D2D target. Call when the monitor changes size
+    // (DPI change, mode change). Leaves Sim intact; caller may regenerate it.
+    bool Resize(int widthPx, int heightPx, UINT dpi);
+
+    // Regenerate the blade layout for the current (post-Resize) DIP width after
+    // a DPI change, reseeding with the same deterministic per-monitor seed and
+    // preserving scene/critter/cut state. Mirrors the Win2D rebuild path. Must
+    // NOT be called on device-loss recovery (which leaves the Sim untouched).
+    void RegenerateForDpi(uint64_t seed, double density);
+
+    // Advance the simulation by `dt` seconds, then draw a frame.
+    void RenderFrame(double dt,
+                     const InputEvent* events,
+                     std::size_t numEvents);
+
+    // For windows that have been minimized / occluded: skip rendering but keep
+    // the simulation alive.
+    void Tick(double dt,
+              const InputEvent* events,
+              std::size_t numEvents);
+
+    Sim&        GetSim()        { return sim_; }
+    const Sim&  GetSim() const  { return sim_; }
+    HWND        GetHwnd() const { return hwnd_; }
+
+    void SetWindowOriginScreen(int x, int y) { windowOriginScreenX_ = x; windowOriginScreenY_ = y; }
+    int  GetWindowOriginScreenX() const { return windowOriginScreenX_; }
+    int  GetWindowOriginScreenY() const { return windowOriginScreenY_; }
+    int  GetWidthPx() const  { return widthPx_; }
+    int  GetHeightPx() const { return heightPx_; }
+    UINT GetDpi() const      { return dpi_; }
+
+private:
+    template<class T> using ComPtr = Microsoft::WRL::ComPtr<T>;
+
+    void Cleanup();
+    bool CreateDeviceResources();
+    bool CreateSwapChainResources(int widthPx, int heightPx);
+    void DiscardDeviceResources();
+    void DrawGrass(bool treesOnly, bool backgroundTrees);
+    void DrawEntities(const D2D1_POINT_2F* cursorPosition);
+    void DrawButterfly(const Entity& e);
+    void DrawFirefly(const Entity& e);
+    void DrawBird(const Entity& e);
+    void DrawCoral(const Blade& b, float groundY);
+    void DrawFish(const Entity& e);
+    void DrawCat(const Entity& e, const D2D1_POINT_2F* cursorPosition);
+    void DrawBunny(const Entity& e);
+    void DrawHedgehog(const Entity& e);
+    void DrawPetName(const Entity& e, const D2D1_POINT_2F* cursorPosition);
+    bool TryGetCursorPositionDip(D2D1_POINT_2F& cursorPosition) const;
+
+    HWND                                   hwnd_ = nullptr;
+    int                                    widthPx_   = 0;
+    int                                    heightPx_  = 0;
+    UINT                                   dpi_       = 96;
+    int                                    windowOriginScreenX_ = 0;
+    int                                    windowOriginScreenY_ = 0;
+
+    ComPtr<ID3D11Device>                   d3dDevice_;
+    ComPtr<ID3D11DeviceContext>            d3dContext_;
+    ComPtr<IDXGIDevice1>                   dxgiDevice_;
+    ComPtr<IDXGIFactory2>                  dxgiFactory_;
+    ComPtr<IDXGISwapChain1>                swapChain_;
+    ComPtr<ID2D1Factory1>                  d2dFactory_;
+    ComPtr<ID2D1Device>                    d2dDevice_;
+    ComPtr<ID2D1DeviceContext>             d2dContext_;
+    ComPtr<ID2D1Bitmap1>                   d2dTarget_;
+    ComPtr<ID2D1SolidColorBrush>           brushes_[SCENE_COUNT][PALETTE_SIZE];
+    ComPtr<ID2D1SolidColorBrush>           flowerHeadBrushes_[FLOWER_PALETTE_SIZE];
+    ComPtr<ID2D1SolidColorBrush>           mushroomCapBrushes_[MUSHROOM_PALETTE_SIZE];
+    ComPtr<ID2D1SolidColorBrush>           mushroomStemBrush_;
+    ComPtr<ID2D1SolidColorBrush>           cactusBrush_;
+    ComPtr<ID2D1StrokeStyle>               roundStrokeStyle_;
+    ComPtr<ID2D1SolidColorBrush>           tumbleweedBrush_;
+    ComPtr<ID2D1SolidColorBrush>           snowflakeBrush_;
+    ComPtr<ID2D1SolidColorBrush>           leafBrushes_[LEAF_COLOR_COUNT];
+    ComPtr<ID2D1SolidColorBrush>           snowTipBrush_;
+    ComPtr<ID2D1SolidColorBrush>           snowBankShadowBrush_;
+    ComPtr<ID2D1SolidColorBrush>           pineBrush_;
+    ComPtr<ID2D1SolidColorBrush>           pineShadowBrush_;
+    ComPtr<ID2D1SolidColorBrush>           pineHighlightBrush_;
+    ComPtr<ID2D1SolidColorBrush>           birchBarkBrush_;
+    ComPtr<ID2D1SolidColorBrush>           birchMarkBrush_;
+    ComPtr<ID2D1SolidColorBrush>           mapleTrunkBrush_;
+    ComPtr<ID2D1SolidColorBrush>           mapleTrunkDarkBrush_;
+    ComPtr<ID2D1SolidColorBrush>           mapleCanopyBrushes_[MAPLE_CANOPY_COLOR_COUNT];
+    ComPtr<ID2D1SolidColorBrush>           coralBrushes_[CORAL_COLOR_COUNT];
+    ComPtr<ID2D1SolidColorBrush>           bubbleStrokeBrush_;
+    ComPtr<ID2D1SolidColorBrush>           bubbleHighlightBrush_;
+    ComPtr<ID2D1SolidColorBrush>           fishBrushes_[FISH_COLOR_COUNT];
+    ComPtr<ID2D1SolidColorBrush>           fishFinBrush_;
+    ComPtr<ID2D1SolidColorBrush>           sheepBodyBrush_;
+    ComPtr<ID2D1SolidColorBrush>           sheepLegBrush_;
+    ComPtr<ID2D1SolidColorBrush>           sheepFaceBrush_;
+    ComPtr<ID2D1SolidColorBrush>           sheepEarBrush_;
+    ComPtr<ID2D1SolidColorBrush>           sheepInkBrush_;
+    struct CatCoatBrushSet {
+        ComPtr<ID2D1SolidColorBrush> body;
+        ComPtr<ID2D1SolidColorBrush> leg;
+        ComPtr<ID2D1SolidColorBrush> face;
+        ComPtr<ID2D1SolidColorBrush> ear;
+        ComPtr<ID2D1SolidColorBrush> ink;
+    };
+    CatCoatBrushSet catCoatBrushes_[CAT_COAT_VARIANT_COUNT];
+    ComPtr<ID2D1SolidColorBrush>           bunnyBodyBrush_;
+    ComPtr<ID2D1SolidColorBrush>           bunnyBellyBrush_;
+    ComPtr<ID2D1SolidColorBrush>           bunnyEarBrush_;
+    ComPtr<ID2D1SolidColorBrush>           bunnyEarInnerBrush_;
+    ComPtr<ID2D1SolidColorBrush>           bunnyTailBrush_;
+    ComPtr<ID2D1SolidColorBrush>           bunnyEyeBrush_;
+    ComPtr<ID2D1SolidColorBrush>           bunnyNoseBrush_;
+    ComPtr<ID2D1SolidColorBrush>           hedgehogBodyBrush_;
+    ComPtr<ID2D1SolidColorBrush>           hedgehogSpikeBrush_;
+    ComPtr<ID2D1SolidColorBrush>           hedgehogSpikeTipBrush_;
+    ComPtr<ID2D1SolidColorBrush>           hedgehogNoseBrush_;
+    ComPtr<ID2D1SolidColorBrush>           hedgehogEyeBrush_;
+    ComPtr<ID2D1SolidColorBrush>           butterflyBodyBrush_;
+    ComPtr<ID2D1SolidColorBrush>           butterflyWingBrushes_[BUTTERFLY_COLOR_COUNT];
+    ComPtr<ID2D1SolidColorBrush>           butterflyAccentBrushes_[BUTTERFLY_COLOR_COUNT];
+    ComPtr<ID2D1SolidColorBrush>           fireflyBodyBrush_;
+    ComPtr<ID2D1SolidColorBrush>           fireflyGlowBrush_;
+    ComPtr<ID2D1SolidColorBrush>           birdBrush_;
+    ComPtr<ID2D1SolidColorBrush>           petNameBrush_;
+    ComPtr<ID2D1SolidColorBrush>           petNameShadowBrush_;
+    ComPtr<IDWriteFactory>                 dwriteFactory_;
+    ComPtr<IDWriteTextFormat>              petNameTextFormat_;
+
+    ComPtr<IDCompositionDevice>            dcompDevice_;
+    ComPtr<IDCompositionTarget>            dcompTarget_;
+    ComPtr<IDCompositionVisual>            dcompVisual_;
+
+    Sim                                    sim_{};
+    std::unordered_map<uint64_t, double>   petNameLastHover_;
+    bool                                   initialized_ = false;
+};
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/Sim.h

@@ -0,0 +1,404 @@
+// Sim.h
+//
+// Pure data + math: PRNG, blade generation, sway, gust, cut. NO Direct2D, D3D,
+// COM, Win32 UI, or threading dependencies. This is what the unit-test project
+// links against.
+//
+// Mirrors docs/architecture.md §§3-10. Constants live in Constants.h.
+
+#pragma once
+
+#include <cstdint>
+#include <cstddef>
+#include <vector>
+
+#include "Constants.h"
+#include "Persistence.h"
+
+namespace desktopgrass {
+
+// ---------------------------------------------------------------------------
+// PRNG: xorshift64 seeded via SplitMix64. See architecture.md §3.
+// ---------------------------------------------------------------------------
+
+struct Prng {
+    uint64_t state;
+};
+
+uint64_t splitmix64(uint64_t z) noexcept;
+void     prng_init(Prng& p, uint64_t seed) noexcept;
+uint64_t prng_next_u64(Prng& p) noexcept;
+uint32_t prng_next_u32(Prng& p) noexcept;
+double   prng_next_unit(Prng& p) noexcept;
+double   prng_uniform(Prng& p, double lo, double hi) noexcept;
+double   prng_exponential(Prng& p, double lambda) noexcept;
+uint32_t prng_index(Prng& p, uint32_t n) noexcept;
+
+// ---------------------------------------------------------------------------
+// Blade. Layout matches architecture.md §4 — field set, not field order, is
+// load-bearing.
+// ---------------------------------------------------------------------------
+
+struct Blade {
+    // Static (set once at generation).
+    double  baseX;
+    double  height;
+    double  thickness;
+    uint8_t hue;
+    double  swayPhaseOffset;
+    double  stiffness;
+
+    // Runtime.
+    double  cutHeight;
+    double  gustVelocity;
+    double  cutAnimStart;
+    double  cutInitialHeight;
+
+    // Residual normalized height a mowed blade settles at (stubble). Assigned
+    // once at generation from an independent salted PRNG stream so it does not
+    // perturb the static-field draws. Defaults to 0.0 so hand-built `Blade b{}`
+    // fixtures collapse fully to a flat stump exactly as before.
+    double  cutFloor = 0.0;
+
+    // Regrowth (Constants.h §"Regrowth"). regrowDelay / regrowDuration are
+    // assigned once at generation from an independent PRNG stream (so they do
+    // not perturb the static fields' draws). regrowStart is the absolute
+    // globalTime at which the regrow animation begins; -1 means "not
+    // scheduled". When cutAnimStart finishes, advance_cut sets
+    // regrowStart = globalTime + regrowDelay. A click on a regrowing blade
+    // cancels regrowth (clears regrowStart) and restarts the cut from current
+    // cutHeight. In-class initializers keep `Blade b{};` (used by tests &
+    // helpers) in the correct "no regrowth scheduled" state.
+    double  regrowDelay    = 0.0;
+    double  regrowDuration = 0.0;
+    double  regrowStart    = -1.0;
+
+    // Flower (§4, §5, §7). Static, set once at generation from an
+    // independent PRNG stream. isFlower=false means this is an ordinary
+    // grass blade; heightBonus defaults to 1.0 so the L formula in
+    // compute_blade_stroke is a no-op for non-flowers.
+    bool    isFlower            = false;
+    uint8_t flowerHeadColorIdx  = 0;
+    double  flowerHeadRadius    = 0.0;
+    double  heightBonus         = 1.0;
+
+    // Mushroom (PROTOTYPE — Native-only). Static, set once at generation from
+    // a fourth independent PRNG stream. When isMushroom=true the renderer
+    // draws a filled-ellipse cap on a short stem at this slot and skips the
+    // grass blade + flower head. cutHeight still drives cut/regrow animation
+    // for mushrooms (cap+stem shrink/grow linearly with it).
+    bool    isMushroom              = false;
+    uint8_t mushroomCapColorIdx     = 0;
+    double  mushroomCapWidth        = 0.0;   // radius X (DIP)
+    double  mushroomCapHeight       = 0.0;   // radius Y (DIP)
+    double  mushroomStemHeight      = 0.0;   // DIP
+    double  mushroomStemThickness   = 0.0;   // DIP
+
+    // Original Grass-scene slot variants. Desert cacti temporarily replace
+    // flower/mushroom tags; switching back to Grass restores these snapshots.
+    bool    originalIsFlower        = false;
+    bool    originalIsMushroom      = false;
+
+    // Cactus (§14). Desert-only slot-bound blade variant.
+    bool    isCactus                = false;
+    uint8_t cactusType              = 0;     // 0 = column, 1 = single-arm, 2 = saguaro
+    double  cactusHeight            = 0.0;   // DIP
+    double  cactusWidth             = 0.0;   // DIP
+    int8_t  cactusArmSide           = +1;    // -1 or +1 for type 1
+
+    // Pine (§15.1). Winter-only slot-bound blade variant.
+    bool    isPine                  = false;
+    uint8_t pineTierCount           = 0;     // 2..4 (only meaningful for treeVariant == 0)
+    uint8_t treeVariant             = 0;     // 0 = pine, 1 = birch
+    double  pineHeight              = 0.0;   // DIP
+    double  pineWidth               = 0.0;   // DIP, base-tier width (pine) or trunk width (birch)
+    bool    treeBackground          = false; // §15.4 depth layer: true = small/hazy, drawn behind the snowbank
+
+    // Maple (§16.5). Autumn-only slot-bound blade variant.
+    bool    isMaple                 = false;
+    double  mapleHeight             = 0.0;   // DIP
+    double  mapleTrunkWidth         = 0.0;   // DIP
+    double  mapleCanopyRadius       = 0.0;   // DIP
+    uint8_t mapleCanopyColorIdx     = 0;
+    bool    mapleIsBare             = false;
+
+    // Coral (§17). Ocean-only slot-bound blade variant.
+    bool    isCoral                 = false;
+    uint8_t coralType               = 0;     // 0 = fan, 1 = branching, 2 = brain
+    double  coralHeight             = 0.0;   // DIP
+    double  coralWidth              = 0.0;   // DIP
+    uint8_t coralColorIdx           = 0;
+
+    // Leaf puff cooldown (§16.6). Absolute globalTime before which this maple
+    // will not shed another hover-triggered leaf flurry. Runtime-only; default
+    // 0.0 keeps `Blade b{}` fixtures ready to puff immediately.
+    double  leafPuffCooldownEnd     = 0.0;
+
+    // Derived per-frame. Stored on the blade for the renderer to consume; not
+    // part of the persistent state and ignored by snapshot tests.
+    double  effectiveLean;
+};
+
+// ---------------------------------------------------------------------------
+// Stroke (rendering geometry). architecture.md §7.
+// ---------------------------------------------------------------------------
+
+struct Point { double x, y; };
+
+struct Stroke {
+    Point    base;
+    Point    control;
+    Point    tip;
+    double   thickness;
+    uint32_t argb;
+};
+
+Stroke compute_blade_stroke(const Blade& b, double groundY, Scene scene) noexcept;
+
+// ---------------------------------------------------------------------------
+// Input event queue. The renderer drains the OS hook into this struct each
+// frame, then calls sim_tick.
+// ---------------------------------------------------------------------------
+
+enum class EventType : uint8_t {
+    Move  = 0,
+    Click = 1,
+};
+
+struct InputEvent {
+    EventType type;
+    double    x;        // window-local DIP
+    double    y;        // window-local DIP
+    double    time;     // seconds, monotonic
+};
+
+// ---------------------------------------------------------------------------
+// Roaming entities (architecture.md §13.2). Tumbleweeds (Desert §14),
+// snowflakes (Winter §15), sheep (§16), cats (§17), bunnies (§17.5),
+// butterflies (§17.6), fireflies (§17.7), and birds (§17.8) live in sim.entities.
+// The struct fields are shared across kinds; per-kind tick logic branches on `kind`.
+// ---------------------------------------------------------------------------
+
+struct Entity {
+    EntityKind kind          = EntityKind::None;
+    double     x             = 0.0;
+    double     y             = 0.0;
+    double     vx            = 0.0;
+    double     vy            = 0.0;
+    double     size          = 0.0;   // radius (DIP)
+    double     rotation      = 0.0;   // radians
+    double     rotationSpeed = 0.0;   // rad/sec
+    double     age           = 0.0;
+    double     lifetime      = -1.0;  // <= 0 means infinite (respawn-in-place)
+    uint32_t   seed          = 0;
+    // Critter state machine (§16-§18). Sheep and Cat share state bytes;
+    // Cat reuses Hopping semantically as Pouncing. Bunny uses BUNNY_STATE_*.
+    // Values are ignored by tumbleweeds/snowflakes and inert by default.
+    uint8_t    state         = 0;     // critters: see species STATE constants
+    double     stateTimer    = 0.0;   // sec remaining in current state
+    uint8_t    previousState = 0;     // hedgehog: pre-curl state
+    double     previousStateTimer = 0.0; // hedgehog: remaining pre-curl time
+    uint8_t    nameIndex     = 0;     // critters: index into species name pool
+    uint8_t    coatVariantIndex = 0;  // cat: index into CAT_COAT_PALETTES
+
+    // Ambient flyers (§17.6-§17.8). These fields are ignored by grounded pets.
+    double     baseSpeed      = 0.0;
+    double     altitudeAnchor = 0.0;
+    double     phaseY         = 0.0;  // butterflies/fireflies: Y phase; birds: vertical drift phase
+    double     phaseX         = 0.0;  // butterflies/fireflies: X phase; birds: wing phase offset
+    double     blinkPeriod    = 0.0;
+    double     blinkPhase     = 0.0;
+    uint8_t    colorVariant   = 0;
+
+    // Bird flyby (§17.8) transient metadata.
+    double     x0             = 0.0;
+    double     spawnTime      = 0.0;
+    double     formationOffsetAlongFlight = 0.0;
+    double     formationOffsetPerpendicular = 0.0;
+};
+
+// ---------------------------------------------------------------------------
+// Sim — the simulation state for one monitor window.
+// ---------------------------------------------------------------------------
+
+struct Sim {
+    std::vector<Blade> blades;
+    double             globalTime          = 0.0;
+    double             prevCursorX         = 0.0;
+    double             prevCursorTime      = -1.0;
+    double             windowHeight        = STRIP_HEIGHT + HEADROOM;
+
+    // §config sway knobs. Multipliers applied in update_blade_dynamics; default
+    // 1.0 reproduces historical behavior. Set once from config after sim_init and
+    // preserved across scene changes (sim_set_scene does not touch them).
+    double             swaySpeedScale      = 1.0;
+    double             swayAmpScale        = 1.0;
+
+    // Ambient gust scheduler (§8.1). Initialized by sim_init / sim_regenerate.
+    Prng               ambientPrng         = { 0 };
+    double             nextAmbientGustTime = 0.0;
+    double             monitorWidth        = 0.0;
+
+    // Current scene (§13). Default Grass; updated by sim_set_scene.
+    Scene              currentScene        = SCENE_DEFAULT;
+
+    // Roaming entities (§13.2). Desert and Winter add their own scene entities
+    // via sim_set_scene. Pre-reserved to MAX_ENTITIES_PER_MONITOR at init so the
+    // tick path never grows the vector.
+    std::vector<Entity> entities;
+
+    // Per-scene entity-stream seed. Set at sim_init; used by sim_set_scene
+    // to construct the per-kind generator PRNGs.
+    uint64_t           entitySeed          = 0;
+
+    // Persistent tumbleweed stream (§14). Initialized when entering Desert and
+    // consumed by off-edge respawns so replay stays deterministic.
+    Prng               tumbleweedPrng      = { 0 };
+
+    // §15 snowflake emitter (Winter scene only)
+    Prng               snowflakePrng       = { 0 };
+    double             nextSnowflakeSpawnTime = 0.0;
+
+    // §16.5 leaf emitter (Autumn scene only).
+    Prng               leafPrng            = { 0 };
+    double             nextLeafSpawnTime   = 0.0;
+
+    // §16.6 leaf-puff emitter (Autumn scene only). Independent salted stream so
+    // cursor-triggered puffs never perturb the ambient leaf emitter's draws.
+    Prng               leafPuffPrng        = { 0 };
+
+    // §21 snow-puff emitter (Winter scene only). Independent salted stream so
+    // click-triggered powder bursts never perturb the snowflake emitter's draws.
+    Prng               snowPuffPrng        = { 0 };
+
+    // §21.1 snow-drift emitter (Winter scene only). Cursor-move spindrift wisps
+    // share an independent salted stream so they never perturb the click puff or
+    // snowflake draws. A global cooldown keeps the kicked-up powder calm.
+    Prng               snowDriftPrng       = { 0 };
+    double             snowDriftCooldownEnd = 0.0;
+
+    // §17 Ocean emitters. Bubble stream rises from the seafloor; fish stream
+    // maintains the swimmer population. Independent salted streams so neither
+    // perturbs the other's draws or the shared coral generator.
+    Prng               bubblePrng          = { 0 };
+    double             nextBubbleSpawnTime = 0.0;
+    Prng               fishPrng            = { 0 };
+
+    // §17.8 daytime bird-flyby emitter. Transient Grass-only flocks share one
+    // persistent stream and one next-event time across scene switches.
+    Prng               birdFlybyPrng       = { 0 };
+    double             nextBirdFlybyAtTime = 0.0;
+
+    // Critter subsystem (§13.3, §16-§18). Grass-scene critters share one PRNG
+    // stream seeded from entitySeed XOR CRITTER_PRNG_SALT at generation time.
+    // critterCountOverride is 0 for random species count, or a fixed count
+    // capped by PET_COUNT_MAX_PER_MONITOR during legacy single-species generation.
+    CritterKind        currentCritter      = CRITTER_DEFAULT;
+    Prng               critterPrng         = { 0 };
+    int                critterCountOverride = 0;
+};
+
+// Construct a sim with blades generated for the given monitor width, density,
+// and seed. windowHeight defaults to STRIP_HEIGHT + HEADROOM.
+Sim sim_init(uint64_t seed, double monitorWidth, double density = 1.0);
+
+// Re-run generation in place, resetting all runtime state.
+void sim_regenerate(Sim& sim, uint64_t seed, double monitorWidth, double density = 1.0);
+
+// Apply a move event. Updates prevCursorX/prevCursorTime and distributes gust
+// impulse. Caller is responsible for the dt-clamp / cap on cursor speed; this
+// function performs the cap internally per the spec.
+void sim_apply_move(Sim& sim, const InputEvent& e) noexcept;
+
+// Apply a click event. cutBand filtering uses sim.windowHeight as groundY.
+void sim_apply_click(Sim& sim, const InputEvent& e) noexcept;
+
+// Ambient gust application (§8.1). Same impulse kernel as cursor gusts but
+// uses GUST_RADIUS * AMBIENT_GUST_RADIUS_FACTOR and an impulse magnitude
+// parameterised by magFactor (a fraction of a saturated cursor sweep) instead
+// of the cursor-derived velocity. Exposed for unit tests.
+void sim_apply_ambient_gust(Sim& sim, double x, double signDir, double magFactor) noexcept;
+
+// Run the ambient gust scheduler one tick. Exposed for unit tests. Idempotent
+// on idle ticks (zero PRNG draws); fires zero or more puffs depending on how
+// many scheduled fire times sim.globalTime has crossed.
+void sim_tick_ambient_gusts(Sim& sim) noexcept;
+
+// Set the current scene (§13). State-only update — does not regenerate
+// blades or perturb any PRNG stream. Renderer reads sim.currentScene at
+// draw time.
+//
+// Phase-3 amendment (§13.1): in addition to the field assign, this clears
+// sim.entities and dispatches to per-scene generators (none yet — empty
+// generator hooks for Grass / Desert / Winter; Desert and Winter content
+// agents add their generators here).
+void sim_set_scene(Sim& sim, Scene s) noexcept;
+
+// Advance roaming entities by dt (§13.2). Generic per-kind tick: position
+// integration, rotation update, age advance, snowflake horizontal sway,
+// tumbleweed off-edge respawn, snowflake culling. Currently a no-op when
+// sim.entities is empty (which is always until §14/§15 generators run).
+void sim_tick_entities(Sim& sim, double dt) noexcept;
+
+// Critter selection (§16-§18). Removes existing critter-kind entities
+// (preserving scene entities like tumbleweeds/snowflakes), then re-runs the
+// critter generator. CritterKind::None spawns no ground critters; only the
+// ambient flyers (butterflies/fireflies) remain.
+void sim_set_critter(Sim& sim, CritterKind c) noexcept;
+
+// Fixed critter count override (§13.3). n=0 clears to random; positive values
+// are capped at PET_COUNT_MAX_PER_MONITOR during generation.
+void sim_set_critter_count(Sim& sim, int n) noexcept;
+
+double bunny_hop_y_offset(double age, bool startled) noexcept;
+uint8_t bunny_choose_rest_state(Prng& p) noexcept;
+uint8_t hedgehog_choose_rest_state(Prng& p) noexcept;
+double bird_flyby_sample_interval(Prng& p) noexcept;
+void sim_spawn_bird_flyby(Sim& sim) noexcept;
+void sim_tick_bird_flybys(Sim& sim) noexcept;
+
+// Advance the simulation by dt seconds. Drains the provided event list in
+// order, then runs per-blade dynamics + cut animation. Pass numEvents = 0 if
+// no events fired this frame.
+void sim_tick(Sim& sim, double dt,
+              const InputEvent* events, std::size_t numEvents) noexcept;
+
+// Generator used by sim_init / sim_regenerate. Exposed for unit tests.
+void generate_blades(uint64_t seed, double monitorWidth, double density,
+                     std::vector<Blade>& out);
+
+// Desert generators (§14). Exposed for unit tests.
+void generate_cacti_for_desert(Sim& sim) noexcept;
+void generate_tumbleweeds(Sim& sim) noexcept;
+
+// Pine tree generator (§15.1). Iterates blade slots; promotes a small
+// fraction to pines from the PINE_PRNG_SALT stream when entering Winter.
+// Slot-bound and reversed by restore_original_variants on scene exit.
+void generate_pines_for_winter(Sim& sim) noexcept;
+
+// Maple tree generator (§16.5). Iterates blade slots; promotes a small
+// fraction to maples from the MAPLE_PRNG_SALT stream when entering Autumn.
+void generate_maples_for_autumn(Sim& sim) noexcept;
+void generate_coral_for_ocean(Sim& sim) noexcept;
+
+// Per-blade dynamics helper (visible for tests).
+void update_blade_dynamics(Blade& b, double globalTime, double dt,
+                           double swaySpeedScale = 1.0,
+                           double swayAmpScale = 1.0) noexcept;
+void advance_cut(Blade& b, double globalTime) noexcept;
+
+// Persistence helpers. GetCuts stores cut timestamps shifted relative to the
+// current sim time (for example, -20 means the blade was cut 20 seconds ago)
+// so a fresh sim can resume regrowth after restart.
+std::vector<persistence::CutRecord> sim_get_cuts(const Sim& sim);
+void sim_apply_cuts(Sim& sim, const std::vector<persistence::CutRecord>& cuts) noexcept;
+
+// dt clamp helper. Required at the renderer boundary so a long pause does not
+// produce visible artifacts. See architecture.md §10.
+constexpr double DT_MIN = 0.001;
+constexpr double DT_MAX = 0.1;
+constexpr double clamp_dt(double dt) noexcept {
+    return (dt < DT_MIN) ? DT_MIN : (dt > DT_MAX ? DT_MAX : dt);
+}
+
+} // namespace desktopgrass

src/modules/DesktopGrass/DesktopGrass.Native/src/main.cpp

@@ -0,0 +1,65 @@
+// main.cpp
+//
+// Entry point: set up DPI awareness, COM, the App, run the message loop.
+//
+// When the command line contains `--benchmark`, dispatch into the benchmark
+// runner instead of the normal App lifecycle. Production tray/persistence
+// code remains untouched in that path.
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#include <combaseapi.h>
+#include <shellapi.h>
+
+#include <cwchar>
+
+#include "App.h"
+#include "Benchmark.h"
+
+namespace {
+
+bool HasBenchmarkFlag(int argc, wchar_t** argv) {
+    for (int i = 1; i < argc; ++i) {
+        if (argv[i] && _wcsicmp(argv[i], L"--benchmark") == 0) return true;
+    }
+    return false;
+}
+
+} // anonymous
+
+int APIENTRY wWinMain(HINSTANCE hInst, HINSTANCE, LPWSTR, int) {
+    // Per-Monitor V2 DPI awareness. Also declared in the manifest so OSes that
+    // honour the manifest pick it up before WinMain runs.
+    SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2);
+
+    HRESULT hr = CoInitializeEx(nullptr, COINIT_APARTMENTTHREADED);
+    if (FAILED(hr)) {
+        return -1;
+    }
+
+    int argc = 0;
+    wchar_t** argv = CommandLineToArgvW(GetCommandLineW(), &argc);
+
+    int exitCode = 0;
+    if (argv && HasBenchmarkFlag(argc, argv)) {
+        desktopgrass::benchmark::Options opts;
+        if (!desktopgrass::benchmark::ParseOptions(argc, argv, opts)) {
+            if (argv) LocalFree(argv);
+            CoUninitialize();
+            return -3;
+        }
+        exitCode = desktopgrass::benchmark::Run(hInst, opts);
+    } else {
+        desktopgrass::App app;
+        if (!app.Initialize(hInst)) {
+            if (argv) LocalFree(argv);
+            CoUninitialize();
+            return -2;
+        }
+        exitCode = app.Run();
+    }
+
+    if (argv) LocalFree(argv);
+    CoUninitialize();
+    return exitCode;
+}

src/modules/DesktopGrass/DesktopGrass.Native/vcpkg.json

@@ -0,0 +1,6 @@
+{
+  "name": "desktopgrass-native",
+  "version-string": "1.0.0",
+  "description": "DesktopGrass Native (Win32 + Direct2D, C++). Manifest reserved; no vcpkg-managed deps in v1 — vendored where needed.",
+  "dependencies": []
+}

src/modules/ShortcutGuide/ShortcutGuide.Ui/ShortcutGuideXAML/App.xaml

@@ -29,6 +29,20 @@
                 TrueValue="Collapsed" />
             <tkconverters:StringVisibilityConverter x:Name="StringVisibilityConverter" />
             <converters:ShortcutDescriptionToKeysConverter x:Name="ShortcutDescriptionToKeysConverter" />
+
+            <!--
+                Path data for the 4-square Windows logo icon used by the "Windows" nav entry in MainWindow.
+                Sized for a 20x20 viewbox to match the icon slot in CustomNavigationViewStyle.
+                Same geometry as src/settings-ui/Settings.UI/SettingsXAML/Controls/Dashboard/ShortcutConflictWindow.xaml.
+                Defined here because the nav item is created in code-behind.
+            -->
+            <x:String x:Key="WindowsLogoPathData">M9 20H0V11H9V20ZM20 20H11V11H20V20ZM9 9H0V0H9V9ZM20 9H11V0H20V9Z</x:String>
+
+            <!--
+                Path data for the PowerToys app icon (rounded square with menu bar and three dots) used in the apps nav list.
+                Scaled from the 64x64 source SVG to fit a 20x20 viewbox; F0 = even-odd fill so the frame strokes render correctly.
+            -->
+            <x:String x:Key="PowerToysLogoPathData">F0 M17.5 0C18.8807 0 20 1.1193 20 2.5V17.5C20 18.8807 18.8807 20 17.5 20H2.5C1.1193 20 0 18.8807 0 17.5V2.5C0 1.1193 1.1193 0 2.5 0H17.5ZM2.5 1.25C1.8096 1.25 1.25 1.8096 1.25 2.5V17.5C1.25 18.1903 1.8096 18.75 2.5 18.75H17.5C18.1903 18.75 18.75 18.1903 18.75 17.5V2.5C18.75 1.8096 18.1903 1.25 17.5 1.25H2.5ZM3.75 15a1.25 1.25 0 1 0 2.5 0a1.25 1.25 0 1 0 -2.5 0ZM8.75 15a1.25 1.25 0 1 0 2.5 0a1.25 1.25 0 1 0 -2.5 0ZM13.75 15a1.25 1.25 0 1 0 2.5 0a1.25 1.25 0 1 0 -2.5 0ZM15 3.75C15.6903 3.75 16.25 4.3097 16.25 5V10C16.25 10.6903 15.6903 11.25 15 11.25H5C4.3097 11.25 3.75 10.6903 3.75 10V5C3.75 4.3097 4.3097 3.75 5 3.75H15ZM5 5V10H15V5H5Z</x:String>
         </ResourceDictionary>
     </Application.Resources>
 </Application>

src/modules/ShortcutGuide/ShortcutGuide.Ui/ShortcutGuideXAML/MainWindow.xaml

@@ -38,13 +38,6 @@
                 Style="{StaticResource RailNavigationViewStyle}">
                 <NavigationView.MenuItems />
                 <NavigationView.FooterMenuItems>
-                    <!--  If footer only has one item, a visual bug can occur. This fake settings button will have set height to zero as soon as the content is loaded  -->
-                    <NavigationViewItem
-                        x:Name="FakeSettingsButton"
-                        Icon="Setting"
-                        SelectsOnInvoked="False"
-                        Tag="Settings"
-                        Tapped="Settings_Tapped" />
                     <NavigationViewItem
                         x:Uid="SettingsButton"
                         Icon="Setting"

src/modules/ShortcutGuide/ShortcutGuide.Ui/ShortcutGuideXAML/MainWindow.xaml.cs

@@ -15,6 +15,8 @@ using Microsoft.UI.Windowing;
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Input;
+using Microsoft.UI.Xaml.Markup;
+using Microsoft.UI.Xaml.Media;
 using Microsoft.UI.Xaml.Media.Imaging;
 using ShortcutGuide.Helpers;
 using ShortcutGuide.Models;
@@ -136,12 +138,6 @@ namespace ShortcutGuide
             // The code below sets the position of the window to the center of the monitor, but only if it hasn't been set before.
             if (!this._setPosition)
             {
-                Content.GettingFocus += (_, _) =>
-                {
-                    this.FakeSettingsButton.Height = 10;
-                    this.FakeSettingsButton.Height = 0;
-                };
-
                 this.SetWindowPosition();
                 this._setPosition = true;
 
@@ -186,7 +182,13 @@ namespace ShortcutGuide
                 {
                     if (item == defaultShellName)
                     {
-                        this.WindowSelector.MenuItems.Add(new NavigationViewItem { Name = item, Content = "Windows", Icon = new FontIcon() { Glyph = "\xE770" } });
+                        var pathData = (string)Application.Current.Resources["WindowsLogoPathData"];
+                        this.WindowSelector.MenuItems.Add(new NavigationViewItem { Name = item, Content = "Windows", Icon = CreatePathIcon(pathData) });
+                    }
+                    else if (item == "Microsoft.PowerToys")
+                    {
+                        var pathData = (string)Application.Current.Resources["PowerToysLogoPathData"];
+                        this.WindowSelector.MenuItems.Add(new NavigationViewItem { Name = item, Content = ManifestInterpreter.GetShortcutsOfApplication(item).Name, Icon = CreatePathIcon(pathData) });
                     }
                     else
                     {
@@ -220,6 +222,17 @@ namespace ShortcutGuide
             return new FontIcon { Glyph = "\uEB91" };
         }
 
+        private static PathIcon CreatePathIcon(string pathData)
+        {
+            var geometry = (Geometry)XamlBindingHelper.ConvertValue(typeof(Geometry), pathData);
+            return new PathIcon
+            {
+                Data = geometry,
+                Width = 20,
+                Height = 20,
+            };
+        }
+
         private bool _hasMovedToRightMonitor;
 
         private void SetWindowPosition()

src/modules/ShortcutGuide/ShortcutGuide.Ui/ShortcutGuideXAML/Styles/CustomNavigationViewStyle.xaml

@@ -800,9 +800,10 @@
                                 MinHeight="{ThemeResource RailNavigationViewItemOnLeftMinHeight}"
                                 RowSpacing="6">
                                 <!--  Store: Added Grid.RowDefinitions & Removed Grid.ColumnDefinitions  -->
+                                <!--  Row 0 fixed height tuned so the 20px icon vertically aligns with the selection pill center (~y=28).  -->
                                 <Grid.RowDefinitions>
-                                    <RowDefinition />
-                                    <RowDefinition />
+                                    <RowDefinition Height="12" />
+                                    <RowDefinition Height="Auto" />
                                     <RowDefinition Height="Auto" />
                                     <RowDefinition />
                                 </Grid.RowDefinitions>

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Dock/DockViewModel.cs

@@ -997,6 +997,9 @@ public sealed partial class DockViewModel : IDisposable
     {
         var isDockEnabled = _settingsService.Settings.EnableDock;
         var dockSide = isDockEnabled ? GetEffectiveSide().ToString().ToLowerInvariant() : "none";
+        /*
+        // TODO: re-enable the collection of active bands at some point, if
+        // we ever get data flowing again.
 
         var (activeStart, activeCenter, activeEnd) = GetActiveBands();
 
@@ -1006,7 +1009,10 @@ public sealed partial class DockViewModel : IDisposable
         var startBands = isDockEnabled ? FormatBands(activeStart) : string.Empty;
         var centerBands = isDockEnabled ? FormatBands(activeCenter) : string.Empty;
         var endBands = isDockEnabled ? FormatBands(activeEnd) : string.Empty;
-
+        */
+        var startBands = string.Empty;
+        var centerBands = string.Empty;
+        var endBands = string.Empty;
         WeakReferenceMessenger.Default.Send(new TelemetryDockConfigurationMessage(
             isDockEnabled, dockSide, startBands, centerBands, endBands));
     }

src/modules/cmdpal/Microsoft.CmdPal.UI/Events/CmdPalDockConfiguration.cs

@@ -51,9 +51,13 @@ public class CmdPalDockConfiguration : EventBase, IEvent
         EventName = "CmdPal_DockConfiguration";
         IsDockEnabled = isDockEnabled;
         DockSide = dockSide;
-        StartBands = startBands;
-        CenterBands = centerBands;
-        EndBands = endBands;
+
+        // TODO: We're not seeing this event in the backend. As a guess, we're
+        // gonna try disabling the data about the enabled bands, just to see
+        // if that helps.
+        StartBands = string.Empty; // startBands;
+        CenterBands = string.Empty; // centerBands;
+        EndBands = string.Empty; // endBands;
     }
 
     public PartA_PrivTags PartA_PrivTags => PartA_PrivTags.ProductAndServiceUsage;

src/settings-ui/Settings.UI.Library/EnabledModules.cs

@@ -71,7 +71,7 @@ namespace Microsoft.PowerToys.Settings.UI.Library
             }
         }
 
-        private bool shortcutGuide = true;
+        private bool shortcutGuide; // defaulting to off
 
         [JsonPropertyName("Shortcut Guide")]
         public bool ShortcutGuide

src/settings-ui/Settings.UI.Library/MonitorInfo.cs

@@ -675,6 +675,51 @@ namespace Microsoft.PowerToys.Settings.UI.Library
             return string.Join(System.Environment.NewLine, lines);
         }
 
+        public string GetDiagnosticsAsText()
+        {
+            var lines = new List<string>();
+
+            lines.Add("Power Display monitor diagnostics");
+            lines.Add($"Generated: {DateTimeOffset.Now:u}");
+            lines.Add(string.Empty);
+
+            lines.Add("Monitor");
+            lines.Add(new string('-', 50));
+            lines.Add($"Name: {Name}");
+            lines.Add($"Display name: {DisplayName}");
+            lines.Add($"Monitor number: {MonitorNumber}");
+            lines.Add($"Monitor ID: {Id}");
+            lines.Add($"Communication method: {CommunicationMethod}");
+            lines.Add(string.Empty);
+
+            lines.Add("Current values");
+            lines.Add(new string('-', 50));
+            lines.Add($"Brightness: {CurrentBrightness}");
+            lines.Add($"Contrast: {Contrast}");
+            lines.Add($"Volume: {Volume}");
+            lines.Add($"Color temperature VCP: 0x{ColorTemperatureVcp:X2}");
+            lines.Add(string.Empty);
+
+            lines.Add("Detected support");
+            lines.Add(new string('-', 50));
+            lines.Add($"Supports brightness: {SupportsBrightness}");
+            lines.Add($"Supports contrast: {SupportsContrast}");
+            lines.Add($"Supports volume: {SupportsVolume}");
+            lines.Add($"Supports input source: {SupportsInputSource}");
+            lines.Add($"Supports color temperature: {SupportsColorTemperature}");
+            lines.Add($"Supports power state: {SupportsPowerState}");
+            lines.Add(string.Empty);
+
+            lines.Add("Raw capabilities");
+            lines.Add(new string('-', 50));
+            lines.Add(string.IsNullOrWhiteSpace(CapabilitiesRaw) ? "No raw capabilities detected" : CapabilitiesRaw);
+            lines.Add(string.Empty);
+
+            lines.Add(GetVcpCodesAsText());
+
+            return string.Join(Environment.NewLine, lines);
+        }
+
         /// <summary>
         /// Update this monitor's properties from another MonitorInfo instance.
         /// This preserves the object reference while updating all properties.

src/settings-ui/Settings.UI.UnitTests/ViewModelTests/General.cs

@@ -349,7 +349,7 @@ namespace ViewModelTests
             Assert.IsTrue(modules.FancyZones);
             Assert.IsTrue(modules.ImageResizer);
             Assert.IsTrue(modules.PowerPreview);
-            Assert.IsTrue(modules.ShortcutGuide);
+            Assert.IsFalse(modules.ShortcutGuide);
             Assert.IsTrue(modules.PowerRename);
             Assert.IsFalse(modules.PowerLauncher);
             Assert.IsTrue(modules.ColorPicker);

src/settings-ui/Settings.UI/SettingsXAML/Views/PowerDisplayPage.xaml

@@ -62,7 +62,6 @@
                                 <ToggleSwitch IsOn="{x:Bind ViewModel.IsEnabled, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
                             </tkcontrols:SettingsCard>
                         </controls:GPOInfoControl>
-
                         <controls:SettingsGroup x:Uid="PowerDisplayGroupHeader" IsEnabled="{x:Bind ViewModel.IsEnabled, Mode=OneWay}">
                             <tkcontrols:SettingsExpander
                                 x:Uid="PowerDisplayFlyoutHeader"
@@ -264,6 +263,17 @@
                                                 <tkcontrols:SettingsCard ContentAlignment="Left">
                                                     <CheckBox x:Uid="PowerDisplay_Monitor_HideMonitor" IsChecked="{x:Bind IsHidden, Mode=TwoWay}" />
                                                 </tkcontrols:SettingsCard>
+                                                <tkcontrols:SettingsCard x:Uid="PowerDisplay_Monitor_Diagnostics">
+                                                    <Button
+                                                        x:Uid="PowerDisplay_Monitor_Diagnostics_CopyButton"
+                                                        Click="CopyMonitorDiagnostics_Click"
+                                                        Tag="{x:Bind}">
+                                                        <StackPanel Orientation="Horizontal" Spacing="{StaticResource PowerDisplayInlineIconSpacing}">
+                                                            <FontIcon FontSize="{StaticResource PowerDisplayPrimaryGlyphFontSize}" Glyph="&#xE8C8;" />
+                                                            <TextBlock x:Uid="PowerDisplay_Monitor_Diagnostics_CopyText" />
+                                                        </StackPanel>
+                                                    </Button>
+                                                </tkcontrols:SettingsCard>
 
                                                 <!--  VCP Capabilities  -->
                                                 <tkcontrols:SettingsCard x:Uid="PowerDisplay_Monitor_VcpCapabilities" Visibility="{x:Bind HasCapabilities, Mode=OneWay, Converter={StaticResource BoolToVisibilityConverter}}">
@@ -349,4 +359,4 @@
             <controls:PageLink x:Uid="LearnMore_PowerDisplay" Link="https://aka.ms/PowerToysOverview_PowerDisplay" />
         </controls:SettingsPageControl.PrimaryLinks>
     </controls:SettingsPageControl>
-</local:NavigablePage>
+</local:NavigablePage>

src/settings-ui/Settings.UI/SettingsXAML/Views/PowerDisplayPage.xaml.cs

@@ -57,6 +57,18 @@ namespace Microsoft.PowerToys.Settings.UI.Views
             }
         }
 
+        private void CopyMonitorDiagnostics_Click(object sender, RoutedEventArgs e)
+        {
+            if (sender is Button button && button.Tag is MonitorInfo monitor)
+            {
+                var diagnosticsText = monitor.GetDiagnosticsAsText();
+
+                var dataPackage = new DataPackage();
+                dataPackage.SetText(diagnosticsText);
+                Clipboard.SetContent(dataPackage);
+            }
+        }
+
         // Profile button event handlers
         private void ProfileButton_Click(object sender, RoutedEventArgs e)
         {

src/settings-ui/Settings.UI/Strings/en-us/Resources.resw

@@ -5265,6 +5265,15 @@ The break timer font matches the text font.</value>
   <data name="LightSwitch_PowerDisplayDisabledWarningBar.Message" xml:space="preserve">
     <value />
   </data>
+  <data name="PowerDisplay_Monitor_Diagnostics.Header" xml:space="preserve">
+    <value>Monitor diagnostics</value>
+  </data>
+  <data name="PowerDisplay_Monitor_Diagnostics.Description" xml:space="preserve">
+    <value>Copy monitor details, detected capabilities, and VCP codes to the clipboard for troubleshooting.</value>
+  </data>
+  <data name="PowerDisplay_Monitor_Diagnostics_CopyText.Text" xml:space="preserve">
+    <value>Copy diagnostics</value>
+  </data>
   <data name="LightSwitch_ApplyMonitorSettingsExpander.Header" xml:space="preserve">
     <value>Apply monitor settings to</value>
   </data>