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>

tests/AcDream.App.Tests/UI/Layout/ToolbarControllerTests.cs

@@ -89,8 +89,8 @@ public class ToolbarControllerTests
 
         ToolbarController.Bind(layout, repo, () => shortcuts,
             iconIds: (_,_,_,_,_) => 0x77u, useItem: g => used = g);
-        // UiEvent is a positional record struct: (SourceId, Target, Type, Data0..3, Payload)
-        slots[Row1[0]].Cell.OnEvent(new UiEvent(0u, null, UiEventType.MouseDown));
+        // Use now fires on Click (mouse-up), not MouseDown — drag/click disambiguation.
+        slots[Row1[0]].Cell.OnEvent(new UiEvent(0u, null, UiEventType.Click));
 
         Assert.Equal(0x5001u, used);
     }