feat(D.5.1): UiItemList widget + factory branch for class 0x10000031

Ports retail UIElement_ItemList (class 0x10000031) as a behavioral-leaf
container that owns its UiItemSlot children procedurally. Single-cell
default covers every toolbar slot; N-cell grid is deferred to the inventory
phase. OnDraw syncs the cell rect to the list's Width/Height each frame so
the cell is sized and hit-testable from the first rendered frame, even
though the factory sets rect AFTER construction.

Factory: adds `0x10000031u => new UiItemList(resolve)` arm before the
fallback, so all 18 toolbar itemlist slots route to UiItemList instead of
UiDatElement.

Tests: 4 new (IsLeafWidget, StartsWithOneCell, Cell_returnsFirstSlot,
Create_buildsUiItemList_forItemListClassId). All 4 pass; full suite green
(415 pass / 2 skip in App.Tests; 0 fail total).

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

src/AcDream.App/UI/Layout/DatWidgetFactory.cs

@@ -67,6 +67,7 @@ public static class DatWidgetFactory
             7  => BuildMeter(info, resolve, datFont),  // UIElement_Meter
             11 => new UiScrollbar(),                   // UIElement_Scrollbar (reg :124137)
             12 => BuildText(info, resolve),            // UIElement_Text (reg :115655)
+            0x10000031u => new UiItemList(resolve),   // UIElement_ItemList — toolbar/inventory/paperdoll slots
             _  => new UiDatElement(info, resolve),     // generic fallback (incl. Type 3 chrome/containers)
         };