fix(D.2b): gapless insert on optimistic move — stop the inventory reshuffle

Visual gate: moving an item within a bag reshuffled every item ("the order is
not set"). Root cause: insert-before set ONLY the dragged item's ContainerSlot
to N, colliding with the item already at N; the sort-by-slot tie then reordered
the whole grid on every repaint. Fix: MoveItemOptimistic now does a proper
index-based INSERT (shift the others, renumber 0..N-1 gapless) like retail's
ItemList_InsertItem, and HandleDropRelease uses the target's GRID INDEX
(SlotIndex) as the placement rather than its raw ContainerSlot. Regression test
pins the shifted, gapless order. Full suite green.

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

src/AcDream.App/UI/Layout/InventoryController.cs

@@ -333,7 +333,7 @@ public sealed class InventoryController : IItemListDragHandler
         {
             container = EffectiveOpen();
             placement = targetCell.ItemId != 0
-                ? (_objects.Get(targetCell.ItemId)?.ContainerSlot ?? targetCell.SlotIndex)   // insert-before
+                ? targetCell.SlotIndex                                                        // insert-before = the target's GRID INDEX (gapless), not its raw ContainerSlot
                 : _objects.GetContents(container).Count;                                      // first empty = append
         }
         else if (targetList == _containerList || targetList == _topContainer)