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/DragDropSpineTests.cs

@@ -39,4 +39,96 @@ public class DragDropSpineTests
         list.RegisterDragHandler(h);
         Assert.Same(h, list.DragHandler);
     }
+
+    // ── UiItemSlot drag-source payload/ghost ────────────────────────────────
+    [Fact]
+    public void GetDragPayload_emptyCell_isNull()
+        => Assert.Null(new UiItemSlot().GetDragPayload());
+
+    [Fact]
+    public void GetDragPayload_boundCell_snapshotsFields()
+    {
+        var cell = new UiItemSlot { SlotIndex = 4, SourceKind = ItemDragSource.ShortcutBar };
+        cell.SetItem(0x5001u, 0x99u);
+        var p = Assert.IsType<ItemDragPayload>(cell.GetDragPayload());
+        Assert.Equal(0x5001u, p.ObjId);
+        Assert.Equal(ItemDragSource.ShortcutBar, p.SourceKind);
+        Assert.Equal(4, p.SourceSlot);
+        Assert.Same(cell, p.SourceCell);
+    }
+
+    [Fact]
+    public void GetDragGhost_emptyCell_isNull()
+        => Assert.Null(new UiItemSlot().GetDragGhost());
+
+    [Fact]
+    public void GetDragGhost_boundCell_returnsIconTuple()
+    {
+        var cell = new UiItemSlot { Width = 32, Height = 32 };
+        cell.SetItem(0x5001u, 0x99u);
+        var g = cell.GetDragGhost();
+        Assert.NotNull(g);
+        Assert.Equal(0x99u, g!.Value.tex);
+        Assert.Equal(32, g.Value.w);
+        Assert.Equal(32, g.Value.h);
+    }
+
+    // ── cell drop-target: DragEnter overlay + DropReleased dispatch ──────────
+    private static (UiItemList list, UiItemSlot cell, SpyHandler h) ListWithHandler()
+    {
+        var list = new UiItemList(_ => (1u, 1, 1)); // non-zero resolve so overlay draw is harmless
+        var h = new SpyHandler();
+        list.RegisterDragHandler(h);
+        return (list, list.Cell, h);
+    }
+
+    private static ItemDragPayload SomePayload()
+        => new(0x5001u, ItemDragSource.ShortcutBar, 0, new UiItemSlot());
+
+    [Fact]
+    public void DragEnter_setsAcceptOverlay_whenHandlerAccepts()
+    {
+        var (_, cell, h) = ListWithHandler();
+        h.AcceptResult = true;
+        cell.OnEvent(new UiEvent(0u, cell, UiEventType.DragEnter, Payload: SomePayload()));
+        Assert.Equal(UiItemSlot.DragAcceptState.Accept, cell.DragAcceptVisual);
+    }
+
+    [Fact]
+    public void DragEnter_setsRejectOverlay_whenHandlerRejects()
+    {
+        var (_, cell, h) = ListWithHandler();
+        h.AcceptResult = false;
+        cell.OnEvent(new UiEvent(0u, cell, UiEventType.DragEnter, Payload: SomePayload()));
+        Assert.Equal(UiItemSlot.DragAcceptState.Reject, cell.DragAcceptVisual);
+    }
+
+    [Fact]
+    public void DragOver_resetsOverlayToNeutral()
+    {
+        var (_, cell, h) = ListWithHandler();
+        cell.OnEvent(new UiEvent(0u, cell, UiEventType.DragEnter, Payload: SomePayload()));
+        cell.OnEvent(new UiEvent(0u, cell, UiEventType.DragOver, Payload: SomePayload()));
+        Assert.Equal(UiItemSlot.DragAcceptState.None, cell.DragAcceptVisual);
+    }
+
+    [Fact]
+    public void DropReleased_accepted_dispatchesToHandler()
+    {
+        var (list, cell, h) = ListWithHandler();
+        var p = SomePayload();
+        cell.OnEvent(new UiEvent(0u, cell, UiEventType.DropReleased, Data0: 1, Payload: p));
+        Assert.NotNull(h.LastDrop);
+        Assert.Same(list, h.LastDrop!.Value.list);
+        Assert.Same(cell, h.LastDrop.Value.cell);
+        Assert.Same(p, h.LastDrop.Value.payload);
+    }
+
+    [Fact]
+    public void DropReleased_notAccepted_skipsDispatch()
+    {
+        var (_, cell, h) = ListWithHandler();
+        cell.OnEvent(new UiEvent(0u, cell, UiEventType.DropReleased, Data0: 0, Payload: SomePayload()));
+        Assert.Null(h.LastDrop);
+    }
 }

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);
     }