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/Layout/ChatWindowController.cs

@@ -20,9 +20,9 @@ namespace AcDream.App.UI.Layout;
 /// <see cref="ElementInfo"/> tree (which contains everything) and adds the behavioral
 /// widgets as children of their parent container widgets (transcript panel
 /// <c>0x10000010</c> / input bar <c>0x10000013</c>) which ARE created as
-/// <see cref="UiDatElement"/> nodes. The scrollbar track (<c>0x10000012</c>) and
-/// channel menu (<c>0x10000014</c>) are created by the factory and are replaced
-/// with their behavioral counterparts here.
+/// <see cref="UiDatElement"/> nodes. The scrollbar track (<c>0x10000012</c>) is built
+/// directly as a <see cref="UiScrollbar"/> by the factory (Type 11) and is bound in place
+/// here. The channel menu (<c>0x10000014</c>) is still replaced with its behavioral counterpart.
 /// </para>
 /// </summary>
 public sealed class ChatWindowController
@@ -71,7 +71,7 @@ public sealed class ChatWindowController
     public UiChatInput Input { get; private set; } = null!;
 
     /// <summary>Scrollbar widget, driven by <see cref="Transcript"/>'s scroll model.</summary>
-    public UiChatScrollbar Scrollbar { get; private set; } = null!;
+    public UiScrollbar Scrollbar { get; private set; } = null!;
 
     /// <summary>Channel-selector menu widget.</summary>
     public UiChannelMenu Menu { get; private set; } = null!;
@@ -110,7 +110,7 @@ public sealed class ChatWindowController
     /// <param name="debugFont">Fallback debug bitmap font (used when
     ///   <paramref name="datFont"/> is null).</param>
     /// <param name="resolve">Dat RenderSurface id → (GL tex handle, px width, px height).
-    ///   Forwarded to <see cref="UiChatScrollbar"/> and <see cref="UiChannelMenu"/>.</param>
+    ///   Forwarded to <see cref="UiScrollbar"/> and <see cref="UiChannelMenu"/>.</param>
     public static ChatWindowController? Bind(
         ElementInfo rootInfo,
         ImportedLayout layout,
@@ -196,33 +196,24 @@ public sealed class ChatWindowController
         inputBar.AddChild(c.Input);
         c.Input.OnSubmit = text => ChatCommandRouter.Submit(text, vm, busProvider(), c._activeChannel);
 
-        // ── Scrollbar — replace the imported track placeholder ────────────
-        // The factory created a UiDatElement for the track. Remove it and place a
-        // behavioral UiChatScrollbar at the same position, driving the transcript's scroll.
+        // ── Scrollbar — bind the factory-built Type-11 track element ────────
+        // The factory now builds the Type-11 track element (0x10000012) as a UiScrollbar
+        // directly. Find it, bind it in place — no remove/add needed.
         var track = layout.FindElement(TrackId);
-        if (track?.Parent is { } trackParent)
+        if (track is UiScrollbar bar)
         {
-            c.Scrollbar = new UiChatScrollbar
-            {
-                // Pull the bar up to the panel top so the top arrow meets the window
-                // border (and lines up with the max/min button at root y=0); the dat
-                // track sits 6px down, which left a gap after the resize-bar reclaim.
-                Left         = track.Left,
-                Top          = 0f,
-                Width        = track.Width,
-                Height       = track.Height + track.Top,
-                Anchors      = track.Anchors,
-                Model        = c.Transcript.Scroll,
-                SpriteResolve = resolve,
-                TrackSprite    = TrackSprite,
-                ThumbSprite    = ThumbSprite,
-                ThumbTopSprite = ThumbTopSprite,
-                ThumbBotSprite = ThumbBotSprite,
-                UpSprite       = UpSprite,
-                DownSprite     = DownSprite,
-            };
-            trackParent.RemoveChild(track);
-            trackParent.AddChild(c.Scrollbar);
+            float oldTop = bar.Top;
+            bar.Top    = 0f;                       // pull up to the panel top (resize-bar reclaim)
+            bar.Height = bar.Height + oldTop;
+            bar.Model         = c.Transcript.Scroll;
+            bar.SpriteResolve = resolve;
+            bar.TrackSprite    = TrackSprite;
+            bar.ThumbSprite    = ThumbSprite;
+            bar.ThumbTopSprite = ThumbTopSprite;
+            bar.ThumbBotSprite = ThumbBotSprite;
+            bar.UpSprite       = UpSprite;
+            bar.DownSprite     = DownSprite;
+            c.Scrollbar = bar;
         }
 
         // ── Channel menu — replace the imported menu placeholder ──────────