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>

src/AcDream.App/UI/UiScrollable.cs

@@ -7,7 +7,7 @@ namespace AcDream.App.UI;
 /// the scroll offset is an integer pixel value (<c>m_iScrollableY</c>) clamped to
 /// [0, ContentHeight - ViewHeight]; the thumb ratio is view/content; the position
 /// ratio is scroll/(content-view). Pure (no GL) so it is fully unit-tested and
-/// shared by the transcript (UiChatView) and the scrollbar (UiChatScrollbar).
+/// shared by the transcript (UiChatView) and the scrollbar (UiScrollbar).
 /// Decomp anchors: SetScrollableXY @0x4740c0, UpdateScrollbarSize_ @0x4741a0,
 /// UpdateScrollbarPosition_ @0x473f20, UIElement_Text::InqScrollDelta @0x4689b0.
 /// </summary>