test(scoped-includes): add tests for variable isolation

Tests verify:
- Legacy mode: vars merged globally (A sees B's VAR, can access UNIQUE_B)
- Scoped mode: vars isolated (A sees own VAR, cannot access UNIQUE_B)
- Inheritance: includes can still access root vars (ROOT_VAR)

Test structure:
- testdata/scoped_includes/ with main Taskfile and two includes
- inc_a and inc_b both define VAR with different values
- Cross-include test shows A trying to access B's UNIQUE_B

testdata/scoped_includes/Taskfile.yml

@@ -0,0 +1,20 @@
+version: "3"
+
+vars:
+  ROOT_VAR: from_root
+
+includes:
+  a: ./inc_a
+  b: ./inc_b
+
+tasks:
+  default:
+    desc: Test scoped includes - vars should be isolated
+    cmds:
+      - task: a:print
+      - task: b:print
+
+  print-root-var:
+    desc: Print ROOT_VAR from root
+    cmds:
+      - echo "ROOT_VAR={{.ROOT_VAR}}"

testdata/scoped_includes/inc_a/Taskfile.yml

@@ -0,0 +1,18 @@
+version: "3"
+
+vars:
+  VAR: value_from_a
+  UNIQUE_A: only_in_a
+
+tasks:
+  print:
+    desc: Print vars from include A
+    cmds:
+      - echo "A:VAR={{.VAR}}"
+      - echo "A:UNIQUE_A={{.UNIQUE_A}}"
+      - echo "A:ROOT_VAR={{.ROOT_VAR}}"
+
+  try-access-b:
+    desc: Try to access B's unique var (should fail in scoped mode)
+    cmds:
+      - echo "A:UNIQUE_B={{.UNIQUE_B}}"

testdata/scoped_includes/inc_b/Taskfile.yml

@@ -0,0 +1,13 @@
+version: "3"
+
+vars:
+  VAR: value_from_b
+  UNIQUE_B: only_in_b
+
+tasks:
+  print:
+    desc: Print vars from include B
+    cmds:
+      - echo "B:VAR={{.VAR}}"
+      - echo "B:UNIQUE_B={{.UNIQUE_B}}"
+      - echo "B:ROOT_VAR={{.ROOT_VAR}}"

testdata/scoped_includes/testdata/TestScopedIncludes-legacy-cross-include.golden

@@ -0,0 +1 @@
+A:UNIQUE_B=only_in_b

testdata/scoped_includes/testdata/TestScopedIncludes-legacy-default.golden

@@ -0,0 +1,6 @@
+A:VAR=value_from_b
+A:UNIQUE_A=only_in_a
+A:ROOT_VAR=from_root
+B:VAR=value_from_b
+B:UNIQUE_B=only_in_b
+B:ROOT_VAR=from_root

testdata/scoped_includes/testdata/TestScopedIncludes-scoped-cross-include.golden

@@ -0,0 +1 @@
+A:UNIQUE_B=

testdata/scoped_includes/testdata/TestScopedIncludes-scoped-default.golden

@@ -0,0 +1,6 @@
+A:VAR=value_from_a
+A:UNIQUE_A=only_in_a
+A:ROOT_VAR=from_root
+B:VAR=value_from_b
+B:UNIQUE_B=only_in_b
+B:ROOT_VAR=from_root

testdata/scoped_includes/testdata/TestScopedIncludes-scoped-inheritance-a.golden

@@ -0,0 +1,3 @@
+A:VAR=value_from_a
+A:UNIQUE_A=only_in_a
+A:ROOT_VAR=from_root

testdata/scoped_includes/testdata/TestScopedIncludes-scoped-isolation-b.golden

@@ -0,0 +1,3 @@
+B:VAR=value_from_b
+B:UNIQUE_B=only_in_b
+B:ROOT_VAR=from_root