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>

docs/architecture/retail-divergence-register.md

@@ -138,7 +138,7 @@ accepted-divergence entries (#96, #49, #50).
 | AP-38 | Chat transcript renders pre-split `ChatLog` lines 1:1; no in-element word-wrap at the panel's current pixel width | `src/AcDream.App/UI/UiChatView.cs` | Retail does in-element wrap via `UIElement_Text::SizeToFit`; our pre-split lines are always shorter than 440 px in practice; a line that overflows clips at the edge rather than wrapping | Very long server system messages (server shutdowns, broadcast announcements) clip rather than wrapping — no information loss, just visual truncation | `UIElement_Text::SizeToFit` @0x467980; `gmMainChatUI` layout |
 | AP-39 | Chat lines carry one color per `ChatKind` (per-line solid color); retail `UIElement_Text` supports per-glyph styled runs (bold, different hue per segment) | `src/AcDream.App/UI/UiChatView.cs:13` | Retail glyph-run parsing lives inside keystone.dll with no PDB/decomp; per-line per-kind coloring is the correct tonal palette and covers all existing chat types | Chat lines retail renders with multiple colors or bold names (e.g. "PlayerName says: text") render as one flat color; subtle visual difference but functionally complete | `UIElement_Text` glyph-run styling (keystone.dll, no decomp) |
 | AP-40 | Single default translucency for the chat window chrome; no focused/unfocused opacity transition; dat font face/size taken from the vitals `vitalsDatFont` (same dat font, not a chat-specific size lookup) | `src/AcDream.App/Rendering/GameWindow.cs` (chatController binding line) | Retail fades the chat window to ~80% alpha when unfocused (`gmMainChatUI::UpdateAlpha @0x4cdea0`); the opacity animation deferred to the Plan-2 window-manager input integration; sharing `vitalsDatFont` is safe — retail uses the same AC-default font for both | The chat window is always fully opaque/same-font rather than subtly fading when idle; no wrong text, but the focused/unfocused breathing rhythm is absent | `gmMainChatUI::UpdateAlpha` @0x4cdea0; `UCF::SetAceFont @0x4d3940` |
-| AP-41 | Scrollbar thumb drawn as a single stretched sprite (`0x06004C63`, the 3-slice middle tile) instead of retail's 3-slice: top cap `0x06004C60` + tiled middle `0x06004C63` + bottom cap `0x06004C66` | `src/AcDream.App/UI/UiChatScrollbar.cs:37` | The middle tile stretches acceptably at chat-panel dimensions; the 3-slice port is a Task-H upgrade acknowledged inline in the `ThumbSprite` property comment | The thumb's top and bottom edges lack the retail end-cap sprites — slightly wrong visual shape at small thumb sizes (thumb too-short for the middle tile to cleanly scale) | `UIElement_Scrollbar::UpdateLayout @0x4710d0`; cap sprites `0x06004C60` (top) + `0x06004C66` (bottom) from base layout `0x2100003E` |
+| AP-41 | Scrollbar thumb 3-slice cap fallback only: single-tile draw (`0x06004C63`) used only when `ThumbTopSprite`/`ThumbBotSprite` are unset; the chat controller passes all three cap ids so the 3-slice path is drawn in practice | `src/AcDream.App/UI/UiScrollbar.cs:35` | The fallback single-tile path is unreachable when caps are bound (chat controller always sets them); the 3-slice path is the active code path | Only if a future caller omits the cap ids will the fallback fire — no visual regression in the chat window | `UIElement_Scrollbar::UpdateLayout @0x4710d0`; cap sprites `0x06004C60` (top) + `0x06004C66` (bottom) from base layout `0x2100003E` |
 
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