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>

tests/AcDream.App.Tests/UI/Layout/ChatLayoutConformanceTests.cs

@@ -0,0 +1,46 @@
+using AcDream.App.UI.Layout;
+
+namespace AcDream.App.Tests.UI.Layout;
+
+/// <summary>
+/// Dat-free conformance tests for the committed chat_21000006.json golden fixture.
+/// Verifies that LayoutImporter.ImportInfos correctly resolves the BaseElement /
+/// BaseLayoutId inheritance chain for the chat window (LayoutDesc 0x21000006).
+/// </summary>
+public class ChatLayoutConformanceTests
+{
+    private static ElementInfo? Find(ElementInfo n, uint id)
+    {
+        if (n.Id == id) return n;
+        foreach (var c in n.Children)
+        {
+            var f = Find(c, id);
+            if (f is not null) return f;
+        }
+        return null;
+    }
+
+    [Fact]
+    public void ChatFixture_ResolvesKnownElements()
+    {
+        var root = FixtureLoader.LoadChatInfos();
+        Assert.NotNull(Find(root, 0x10000011u)); // transcript
+        Assert.NotNull(Find(root, 0x10000016u)); // input
+        Assert.NotNull(Find(root, 0x10000012u)); // scrollbar track
+        Assert.NotNull(Find(root, 0x10000014u)); // channel menu
+        Assert.NotNull(Find(root, 0x10000019u)); // send button
+        Assert.NotNull(Find(root, 0x1000046Fu)); // max/min button
+    }
+
+    [Fact]
+    public void ChatFixture_ResolvedTypes_MatchRetailRegistry()
+    {
+        var root = FixtureLoader.LoadChatInfos();
+        Assert.Equal(6u,  Find(root, 0x10000014u)!.Type);  // Menu
+        Assert.Equal(11u, Find(root, 0x10000012u)!.Type);  // Scrollbar
+        Assert.Equal(1u,  Find(root, 0x10000019u)!.Type);  // Button (Send)
+        Assert.Equal(1u,  Find(root, 0x1000046Fu)!.Type);  // Button (Max/Min)
+        Assert.Equal(12u, Find(root, 0x10000011u)!.Type);  // Text/style-prototype (transcript)
+        Assert.Equal(12u, Find(root, 0x10000016u)!.Type);  // Text/style-prototype (input)
+    }
+}