feat(D.5.1): UiItemSlot widget (UIElement_UIItem cell port)

Behavioral leaf widget for the toolbar item cell. Draws the empty-slot
sprite (0x060074CF) when unbound; draws the pre-composited icon texture
when a weenie is bound via SetItem(). ConsumesDatChildren=true prevents
the LayoutImporter from double-building the dat sub-elements. SpriteResolve
is configurable so paperdoll equip slots can swap in per-slot silhouettes
later. No Clicked/OnEvent — that wiring comes in Task 8 (ToolbarController).

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

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

@@ -0,0 +1,31 @@
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiItemSlotTests
+{
+    [Fact]
+    public void IsLeafWidget()
+        => Assert.True(new UiItemSlot().ConsumesDatChildren);
+
+    [Fact]
+    public void DefaultEmptySprite_isToolbarBorder()
+        => Assert.Equal(0x060074CFu, new UiItemSlot().EmptySprite);
+
+    [Fact]
+    public void Empty_whenNoItem()
+    {
+        var s = new UiItemSlot();
+        Assert.Equal(0u, s.ItemId);
+        Assert.Equal(0u, s.IconTexture);
+    }
+
+    [Fact]
+    public void SetItem_setsIdAndTexture()
+    {
+        var s = new UiItemSlot();
+        s.SetItem(0x5001u, 0x99u);
+        Assert.Equal(0x5001u, s.ItemId);
+        Assert.Equal(0x99u, s.IconTexture);
+    }
+}