feat(ui): D.5.3/B.1 — UiRoot payload injection + cursor drag ghost (AP-47)

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

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

@@ -131,4 +131,76 @@ public class DragDropSpineTests
         cell.OnEvent(new UiEvent(0u, cell, UiEventType.DropReleased, Data0: 0, Payload: SomePayload()));
         Assert.Null(h.LastDrop);
     }
+
+    // ── Full UiRoot chain: arming + use-vs-drag ─────────────────────────────
+    // A bound, hit-testable slot inside a list, sized for the hit-test.
+    private static (UiRoot root, UiItemList list, UiItemSlot cell) RootWithBoundSlot(uint itemId)
+    {
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var list = new UiItemList(_ => (1u, 1, 1)) { Left = 0, Top = 0, Width = 32, Height = 32 };
+        // Tests don't run OnDraw (which sizes the cell), so size the cell explicitly.
+        list.Cell.Width = 32; list.Cell.Height = 32;
+        if (itemId != 0) list.Cell.SetItem(itemId, 0x99u);
+        root.AddChild(list);
+        return (root, list, list.Cell);
+    }
+
+    [Fact]
+    public void BeginDrag_arms_whenPayloadNonNull()
+    {
+        var (root, _, cell) = RootWithBoundSlot(0x5001u);
+        root.OnMouseDown(UiMouseButton.Left, 10, 10);
+        root.OnMouseMove(20, 10);                  // >3px → promote to drag
+        Assert.Same(cell, root.DragSource);
+        Assert.IsType<ItemDragPayload>(root.DragPayload);
+    }
+
+    [Fact]
+    public void BeginDrag_doesNotArm_whenPayloadNull_emptySlot()
+    {
+        var (root, _, _) = RootWithBoundSlot(0u);   // empty cell → GetDragPayload null
+        root.OnMouseDown(UiMouseButton.Left, 10, 10);
+        root.OnMouseMove(20, 10);
+        Assert.Null(root.DragSource);               // never armed
+    }
+
+    [Fact]
+    public void Click_withoutDrag_firesUse()
+    {
+        var (root, _, cell) = RootWithBoundSlot(0x5001u);
+        bool used = false;
+        cell.Clicked = () => used = true;
+        root.OnMouseDown(UiMouseButton.Left, 10, 10);
+        root.OnMouseUp(UiMouseButton.Left, 10, 10); // no move → Click emitted
+        Assert.True(used);
+    }
+
+    [Fact]
+    public void CompletedDrag_doesNotFireUse()
+    {
+        var (root, _, cell) = RootWithBoundSlot(0x5001u);
+        bool used = false;
+        cell.Clicked = () => used = true;
+        root.OnMouseDown(UiMouseButton.Left, 10, 10);
+        root.OnMouseMove(20, 10);                   // promote to drag
+        root.OnMouseUp(UiMouseButton.Left, 20, 10); // FinishDrag, NOT Click
+        Assert.False(used);
+    }
+
+    // ── no-handler / orphan-cell DragEnter defaults to Reject (review carry-forward) ──
+    [Fact]
+    public void DragEnter_orphanCell_noList_defaultsToReject()
+    {
+        var cell = new UiItemSlot();   // no parent list → FindList() null
+        cell.OnEvent(new UiEvent(0u, cell, UiEventType.DragEnter, Payload: SomePayload()));
+        Assert.Equal(UiItemSlot.DragAcceptState.Reject, cell.DragAcceptVisual);
+    }
+
+    [Fact]
+    public void DragEnter_listWithoutHandler_defaultsToReject()
+    {
+        var list = new UiItemList(_ => (1u, 1, 1));   // no RegisterDragHandler
+        list.Cell.OnEvent(new UiEvent(0u, list.Cell, UiEventType.DragEnter, Payload: SomePayload()));
+        Assert.Equal(UiItemSlot.DragAcceptState.Reject, list.Cell.DragAcceptVisual);
+    }
 }