feat(ui): D.5.3/B.1 — UiItemSlot drag source + drop target + accept/reject overlay

- UiElement: two new virtuals GetDragPayload()/GetDragGhost() (default null)
  keep UiRoot item-agnostic; any leaf can opt into drag by overriding these.
- UiItemSlot: SlotIndex + SourceKind properties for payload identity; two
  overrides return ItemDragPayload / icon ghost when the slot is occupied.
  FindList() walks the parent chain to locate the owning UiItemList and its
  registered IItemListDragHandler.
- UiItemSlot.OnEvent: MouseDown now just consumes the press; use-item fires
  on Click (mouse-up) so a drag doesn't also trigger the use-item callback.
  DragEnter → ask handler, set Accept/Reject overlay. DragOver → reset to
  None (fires on leave). DropReleased → clear overlay + dispatch to handler
  when Data0 == 1 (accepted). DragBegin consumed (source).
- OnDraw: accept/reject sprite overlay drawn last, guarded on id != 0 to
  avoid the resolve(0)-→-magenta footgun.
- ToolbarControllerTests: Click_emitsUseForBoundItem changed from MouseDown
  to Click to match the new dispatch.
- 12 new DragDropSpineTests pass; full suite 481/483 (2 pre-existing skips).

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

src/AcDream.App/UI/UiElement.cs

@@ -203,6 +203,16 @@ public abstract class UiElement
     /// </summary>
     public virtual bool OnEvent(in UiEvent e) => false;
 
+    /// <summary>The data this element carries when a drag begins. <see cref="UiRoot"/>
+    /// pulls this on drag-promote; a NULL return CANCELS the drag (retail:
+    /// ItemList_BeginDrag only arms an occupied cell). Default null = not draggable.</summary>
+    public virtual object? GetDragPayload() => null;
+
+    /// <summary>The texture <see cref="UiRoot"/> paints at the cursor while this element
+    /// is the drag source: (GL handle, width, height). Null = no ghost. Keeps
+    /// <see cref="UiRoot"/> item-agnostic. Retail analog: m_dragIcon (decomp 229738).</summary>
+    public virtual (uint tex, int w, int h)? GetDragGhost() => null;
+
     /// <summary>
     /// Tooltip text for this widget. Retail fires event 0x07 after
     /// ~1000ms hover, then queries the widget's virtual "GetString"