CmdPal: Lightning-fast mode (#45764)

## Summary of the Pull Request

This PR unlocks lightning-fast mode for Command Palette:

- Hides visual and motion distractions when updating the result list:
- Ensures the first interactable result item is selected as early as
possible after the result list is updated, reducing flashing and
blinking caused by the selection highlight moving around.
- Removes the list item selection indicator animation (unfortunately by
removing the pill altogether for now) and prevents it from temporarily
appearing on other items as the selection moves.
- Adds a new "Results" section header above the home page results when
no other section is present.
- This ensures the first item on the home page has consistent visuals
and styling, preventing offsets and excessive visual changes when
elements are replaced in place.

- Improves update performance and container reuse:
- Fixes the `removed` output parameter in `ListHelper.UpdateInPlace` to
only include items that were actually removed (items that were merely
moved to a different position should not be reported as removed).
    - Adds unit tests to prevent regression.
- Updates `ListHelper.UpdateInPlace` for `ObservableCollection` to use
`Move` instead of `Remove`/`Add`, and avoids `Clear` to prevent
`ListView` resets (which force recreation of all item containers and are
expensive).
- Adds a simple cache for list page item view models to reduce
unnecessary recreation during forward incremental search.
- `ListViewModel` and `FetchItems` have no notion of item lifetime or
incremental search phase, so the cache intentionally remains simple
rather than clever.
  - Updates ListPage templates to make them a little lighter:
- Tag template uses OneTime, instead of OneWay - since Tag is immutable
- Replaces ItemsControl with ItemsRepeater for Tag list on list items
- Increases the debounce for showing the details pane and adds a
debounce for hiding it. This improves performance when browsing the list
and prevents the details pane animation from bouncing left and right

## Pictures? Moving!



https://github.com/user-attachments/assets/36428d20-cf46-4321-83c0-d94d6d4a2299



<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: #44407
- [x] Closes: #45691

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/MainListPage.cs

@@ -36,6 +36,11 @@ public sealed partial class MainListPage : DynamicListPage,
     private readonly ScoringFunction<IListItem> _fallbackScoringFunction;
     private readonly IFuzzyMatcherProvider _fuzzyMatcherProvider;
 
+    // Stable separator instances so that the VM cache and InPlaceUpdateList
+    // recognise them across successive GetItems() calls
+    private readonly Separator _resultsSeparator = new(Resources.results);
+    private readonly Separator _fallbacksSeparator = new(Resources.fallbacks);
+
     private RoScored<IListItem>[]? _filteredItems;
     private RoScored<IListItem>[]? _filteredApps;
 
@@ -171,9 +176,40 @@ public sealed partial class MainListPage : DynamicListPage,
             // filtered results.
             if (string.IsNullOrWhiteSpace(SearchText))
             {
-                return _tlcManager.TopLevelCommands
-                    .Where(tlc => !tlc.IsFallback && !string.IsNullOrEmpty(tlc.Title))
-                    .ToArray();
+                var allCommands = _tlcManager.TopLevelCommands;
+
+                // First pass: count eligible commands
+                var eligibleCount = 0;
+                for (var i = 0; i < allCommands.Count; i++)
+                {
+                    var cmd = allCommands[i];
+                    if (!cmd.IsFallback && !string.IsNullOrEmpty(cmd.Title))
+                    {
+                        eligibleCount++;
+                    }
+                }
+
+                if (eligibleCount == 0)
+                {
+                    return [];
+                }
+
+                // +1 for the separator
+                var result = new IListItem[eligibleCount + 1];
+                result[0] = _resultsSeparator;
+
+                // Second pass: populate
+                var writeIndex = 1;
+                for (var i = 0; i < allCommands.Count; i++)
+                {
+                    var cmd = allCommands[i];
+                    if (!cmd.IsFallback && !string.IsNullOrEmpty(cmd.Title))
+                    {
+                        result[writeIndex++] = cmd;
+                    }
+                }
+
+                return result;
             }
             else
             {
@@ -190,6 +226,8 @@ public sealed partial class MainListPage : DynamicListPage,
                     validScoredFallbacks,
                     _filteredApps,
                     validFallbacks,
+                    _resultsSeparator,
+                    _fallbacksSeparator,
                     AppResultLimit);
             }
         }