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>

src/AcDream.App/UI/IItemListDragHandler.cs

@@ -0,0 +1,19 @@
+namespace AcDream.App.UI;
+
+/// <summary>
+/// A panel controller implements this and registers itself on each of its
+/// <see cref="UiItemList"/>s. Port of retail's <c>m_dragHandler</c> vtable
+/// (<c>RegisterItemListDragHandler</c>, decomp 230461; confirmed acclient
+/// 0x004a539e + the gmToolbarUI block 0x004bdd89).
+/// <para><see cref="OnDragOver"/> decides the accept/reject OVERLAY only (advisory).
+/// <see cref="HandleDropRelease"/> is authoritative — it performs the action, or
+/// no-ops to reject.</para>
+/// </summary>
+public interface IItemListDragHandler
+{
+    /// <summary>True ⇒ the target cell shows the accept (green) frame; false ⇒ reject (red).</summary>
+    bool OnDragOver(UiItemList targetList, UiItemSlot targetCell, ItemDragPayload payload);
+
+    /// <summary>Perform the drop (issue the per-panel wire action), or no-op to reject.</summary>
+    void HandleDropRelease(UiItemList targetList, UiItemSlot targetCell, ItemDragPayload payload);
+}