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>

tests/AcDream.App.Tests/UI/DragDropSpineTests.cs

@@ -0,0 +1,42 @@
+using AcDream.App.UI;
+using Xunit;
+
+namespace AcDream.App.Tests.UI;
+
+public class DragDropSpineTests
+{
+    // A spy handler used across the spine tests.
+    private sealed class SpyHandler : IItemListDragHandler
+    {
+        public bool AcceptResult = true;
+        public (UiItemList list, UiItemSlot cell, ItemDragPayload payload)? LastOver;
+        public (UiItemList list, UiItemSlot cell, ItemDragPayload payload)? LastDrop;
+
+        public bool OnDragOver(UiItemList list, UiItemSlot cell, ItemDragPayload p)
+        { LastOver = (list, cell, p); return AcceptResult; }
+
+        public void HandleDropRelease(UiItemList list, UiItemSlot cell, ItemDragPayload p)
+        { LastDrop = (list, cell, p); }
+    }
+
+    [Fact]
+    public void Payload_holdsAllFields()
+    {
+        var src = new UiItemSlot();
+        var p = new ItemDragPayload(0x5001u, ItemDragSource.ShortcutBar, 3, src);
+        Assert.Equal(0x5001u, p.ObjId);
+        Assert.Equal(ItemDragSource.ShortcutBar, p.SourceKind);
+        Assert.Equal(3, p.SourceSlot);
+        Assert.Same(src, p.SourceCell);
+    }
+
+    [Fact]
+    public void UiItemList_registerDragHandler_roundtrips()
+    {
+        var list = new UiItemList(_ => (0u, 0, 0));
+        Assert.Null(list.DragHandler);
+        var h = new SpyHandler();
+        list.RegisterDragHandler(h);
+        Assert.Same(h, list.DragHandler);
+    }
+}