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>

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

@@ -0,0 +1,25 @@
+using AcDream.App.UI;
+using Xunit;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiItemListTests
+{
+    [Fact]
+    public void IsLeafWidget() => Assert.True(new UiItemList().ConsumesDatChildren);
+
+    [Fact]
+    public void StartsWithOneCell_forSingleCellSlot()
+    {
+        var list = new UiItemList();
+        Assert.Equal(1, list.GetNumUIItems());
+        Assert.NotNull(list.GetItem(0));
+    }
+
+    [Fact]
+    public void Cell_returnsTheFirstSlot()
+    {
+        var list = new UiItemList();
+        Assert.Same(list.GetItem(0), list.Cell);
+    }
+}