feat(ui): D.2b inventory finish — UiItemList mouse-wheel scroll

OnEvent handles the Scroll wheel in grid mode (mirrors UiText's sign), driving
the shared UiScrollable. The bound gutter scrollbar (next task) is the primary
scroll affordance; the wheel is the convenience path.

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

src/AcDream.App/UI/UiItemList.cs

@@ -129,6 +129,17 @@ public sealed class UiItemList : UiElement
         _cells.Clear();
     }
 
+    public override bool OnEvent(in UiEvent e)
+    {
+        if (e.Type == UiEventType.Scroll && CellWidth > 0f)
+        {
+            // Mirror UiText: Silk +Y wheel = up/older = decrease ScrollY; negate Data0.
+            Scroll.ScrollByLines(-e.Data0);
+            return true;
+        }
+        return base.OnEvent(e);
+    }
+
     protected override void OnDraw(UiRenderContext ctx)
     {
         // The factory sets Width/Height AFTER construction, so re-layout each frame: