feat(ui): D.5.3/B.1 — drag payload + handler interface + UiItemList registration

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

src/AcDream.App/UI/UiItemList.cs

@@ -24,6 +24,14 @@ public sealed class UiItemList : UiElement
 
     public Func<uint, (uint tex, int w, int h)>? SpriteResolve { get; set; }
 
+    /// <summary>The drag handler this list routes drops to (a panel controller).
+    /// Null = the list accepts no drops. Retail: UIElement_ItemList::m_dragHandler.</summary>
+    public IItemListDragHandler? DragHandler { get; private set; }
+
+    /// <summary>Register the panel's drag handler on this list. Retail:
+    /// UIElement_ItemList::RegisterItemListDragHandler (decomp 230461).</summary>
+    public void RegisterDragHandler(IItemListDragHandler handler) => DragHandler = handler;
+
     /// <summary>Convenience for single-cell slots (the toolbar): the first cell.
     /// Valid only while the list has at least one cell; after <see cref="Flush"/>
     /// (the inventory-phase rebuild path) the list is empty until <see cref="AddItem"/>