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/ItemDragPayload.cs

@@ -0,0 +1,24 @@
+namespace AcDream.App.UI;
+
+/// <summary>
+/// Where a dragged item came from — the retail <c>InqDropIconInfo</c> flag
+/// distinction (<c>flags &amp; 0xE == 0</c> fresh-from-inventory vs
+/// <c>flags &amp; 4</c> within-list reorder) expressed as a typed enum. The drop
+/// handler maps SourceKind + target back to the fresh-vs-reorder decision.
+/// Decomp anchors: gmToolbarUI 0x004bd162 / 0x004bd1af; InqDropIconInfo 230533.
+/// </summary>
+public enum ItemDragSource { Inventory, ShortcutBar, Equipment, Ground }
+
+/// <summary>
+/// Snapshot of a drag-in-progress, taken at drag-begin (so a server move arriving
+/// mid-drag can't mutate it under us). Port of retail's <c>m_dragElement</c> +
+/// <c>InqDropIconInfo</c> out-params (objId/container/flags, decomp 230533).
+/// <para><c>SourceContainer</c> is intentionally NOT stored: the handler resolves the
+/// LIVE container via <c>ClientObjectTable.Get(ObjId).ContainerId</c> at drop — the
+/// same container id retail reads off the dragged element, single source of truth.</para>
+/// </summary>
+public sealed record ItemDragPayload(
+    uint ObjId,                 // dragged weenie guid (retail itemID, +0x5FC)
+    ItemDragSource SourceKind,  // what kind of slot it left
+    int SourceSlot,             // the source cell's SlotIndex (retail m_lastShortcutNumDragged)
+    UiItemSlot SourceCell);     // back-ref: reorder-restore / clear source state / ghost