test(D.2b): chat golden fixture + resolved-Type conformance (widget-generalization Task 1)

- Add ChatLayoutFixtureGenerator.cs (Skip-by-default) to regenerate
  chat_21000006.json from the live portal.dat via LayoutImporter.ImportInfos
- Commit generated fixture chat_21000006.json (13 KB, 400 lines) — dat-free,
  auto-copied to test output via existing *.json csproj glob
- Refactor FixtureLoader: extract shared LoadInfos(fileName) helper; add
  LoadChat() + LoadChatInfos() mirroring the vitals pattern; LoadVitalsInfos()
  now delegates to the shared loader (behavior unchanged, vitals tests green)
- Add ChatLayoutConformanceTests: ResolvesKnownElements + ResolvedTypes_MatchRetailRegistry

Confirmed resolved Types from live dat:
  0x10000011 (transcript) → Type 12 (style-prototype, skipped by factory)
  0x10000016 (input)      → Type 12 (style-prototype, skipped by factory)
  0x10000014 (menu)       → Type 6
  0x10000012 (scrollbar)  → Type 11
  0x10000019 (send)       → Type 1
  0x1000046F (max/min)    → Type 1

Also fix pre-existing build break: UiChatInput.MoveCaret(int delta) was made
private in ce848c1 but UiChatInputTests.Backspace_DeletesBeforeCaret called it
as public. Expose a public MoveCaret(int) overload (no-shift) alongside the
private MoveCaret(int,bool) — restores the intended test surface.

Full suite: 398 passed, 2 skipped (generator + pre-existing), 0 failed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/UI/UiChatInput.cs

@@ -101,6 +101,10 @@ public sealed class UiChatInput : UiElement
         _historyIndex = -1;
     }
 
+    /// <summary>Move the caret left (negative) or right (positive) by <paramref name="delta"/>
+    /// glyph positions without extending a selection. Public for test access.</summary>
+    public void MoveCaret(int delta) => MoveCaretTo(_caret + delta, false);
+
     private void MoveCaret(int delta, bool shift) => MoveCaretTo(_caret + delta, shift);
 
     // ── Selection ────────────────────────────────────────────────────────