feat(D.2b): UiScrollbar (Type 11) — promote the generic chat scrollbar (widget-generalization Task 2)

- git mv UiChatScrollbar.cs → UiScrollbar.cs; rename class + update doc summary to
  "Generic scrollbar. Ports retail UIElement_Scrollbar (RegisterElementClass(0xb) @
  acclient_2013_pseudo_c.txt:124137); thumb size = trackLen * ThumbRatio (min 8px); step ±1 line."
- git mv UiChatScrollbarTests.cs → UiScrollbarTests.cs; rename test class + replace
  every UiChatScrollbar reference with UiScrollbar (bodies unchanged).
- DatWidgetFactory: register Type 11 → new UiScrollbar() before the _ fallback case.
- ChatWindowController: change Scrollbar property type to UiScrollbar; replace the old
  "construct-remove-add" block with a "find factory-built UiScrollbar and bind in place"
  block (no RemoveChild/AddChild); keep `var track` assignment in scope so the Max/Min
  block's track.Left/track.Width reads still compile against UiElement?.
- AP-41 divergence register: update file:line to UiScrollbar.cs:35; narrow wording to
  "fallback only — single-tile drawn only when cap ids are unset; the chat controller
  passes all three cap ids so the 3-slice path is the active code path."
- Update inline UiChatScrollbar doc-comment references in UiScrollable.cs + UiChatView.cs.
- Full suite: 399 passed, 2 skipped (dat/tower fixture skips), 0 failed.

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

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

@@ -97,6 +97,15 @@ public class DatWidgetFactoryTests
         Assert.Null(DatWidgetFactory.Create(noMedia, NoTex, null));
     }
 
+    // ── Test 5b: Type 11 → UiScrollbar ──────────────────────────────────────
+
+    [Fact]
+    public void Type11_Scrollbar_MakesUiScrollbar()
+    {
+        var e = DatWidgetFactory.Create(new ElementInfo { Type = 11, Width = 16, Height = 68 }, NoTex, null);
+        Assert.IsType<UiScrollbar>(e);
+    }
+
     // ── Test 6: Meter slice extraction (the important one) ───────────────────
 
     /// <summary>

tests/AcDream.App.Tests/UI/UiScrollbarTests.cs

@@ -4,9 +4,9 @@ using Xunit;
 namespace AcDream.App.Tests.UI;
 
 /// <summary>
-/// Pure unit tests for <see cref="UiChatScrollbar.ThumbRect"/> — no GL dependency.
+/// Pure unit tests for <see cref="UiScrollbar.ThumbRect"/> — no GL dependency.
 /// </summary>
-public class UiChatScrollbarTests
+public class UiScrollbarTests
 {
     // Model: content=400, view=100, trackLen=200.
     // ThumbRatio = 100/400 = 0.25  →  thumbH = max(8, 200*0.25) = 50.
@@ -17,7 +17,7 @@ public class UiChatScrollbarTests
     {
         var m = new UiScrollable { ContentHeight = 400, ViewHeight = 100 };
         // PositionRatio = 0 (start).
-        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop: 0f, trackLen: 200f);
+        var (y, h) = UiScrollbar.ThumbRect(m, trackTop: 0f, trackLen: 200f);
         Assert.Equal(50f, h, 3f);
         Assert.Equal(0f, y, 3f);
     }
@@ -28,7 +28,7 @@ public class UiChatScrollbarTests
         var m = new UiScrollable { ContentHeight = 400, ViewHeight = 100 };
         m.ScrollToEnd(); // PositionRatio = 1.
         float trackTop = 16f, trackLen = 200f;
-        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop, trackLen);
+        var (y, h) = UiScrollbar.ThumbRect(m, trackTop, trackLen);
         Assert.Equal(50f, h, 3f);
         // y = trackTop + travel * 1 = 16 + 150 = 166.
         Assert.Equal(166f, y, 3f);
@@ -41,7 +41,7 @@ public class UiChatScrollbarTests
         // thumbH=50; travel=150; y = trackTop + 150 = trackTop + 150.
         var m = new UiScrollable { ContentHeight = 400, ViewHeight = 100 };
         m.ScrollToEnd();
-        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop: 16f, trackLen: 200f);
+        var (y, h) = UiScrollbar.ThumbRect(m, trackTop: 16f, trackLen: 200f);
         Assert.Equal(50f, h, 3f);
         Assert.Equal(166f, y, 3f); // 16 + 150
     }
@@ -54,7 +54,7 @@ public class UiChatScrollbarTests
         m.SetScrollY(150);
         Assert.Equal(0.5f, m.PositionRatio, 3);
 
-        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop: 0f, trackLen: 200f);
+        var (y, h) = UiScrollbar.ThumbRect(m, trackTop: 0f, trackLen: 200f);
         Assert.Equal(50f, h, 3f);
         // y = 0 + 150 * 0.5 = 75.
         Assert.Equal(75f, y, 3f);
@@ -65,7 +65,7 @@ public class UiChatScrollbarTests
     {
         // content=1000, view=10, trackLen=200 → ThumbRatio=0.01 → raw=2 < 8 → clamp to 8.
         var m = new UiScrollable { ContentHeight = 1000, ViewHeight = 10 };
-        var (_, h) = UiChatScrollbar.ThumbRect(m, trackTop: 0f, trackLen: 200f);
+        var (_, h) = UiScrollbar.ThumbRect(m, trackTop: 0f, trackLen: 200f);
         Assert.Equal(8f, h, 3f);
     }
 
@@ -74,7 +74,7 @@ public class UiChatScrollbarTests
     {
         // content <= view → ThumbRatio = 1 → thumbH = trackLen.
         var m = new UiScrollable { ContentHeight = 50, ViewHeight = 100 };
-        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop: 16f, trackLen: 100f);
+        var (y, h) = UiScrollbar.ThumbRect(m, trackTop: 16f, trackLen: 100f);
         Assert.Equal(100f, h, 3f);
         Assert.Equal(16f, y, 3f); // travel = 0 → y = trackTop
     }