fix(items): complete retail corpse looting feedback

Route corpse Use through the shared ItemHolder policy so Stuck corpses open instead of being sent as pickup requests. Restore the framed, horizontally resizable external-container strip and use a compact initial width. Port retail's target-list pending item projection so loot is marked in the chosen inventory slot without changing canonical ownership before the server confirms.

Co-authored-by: OpenAI Codex <codex@openai.com>

docs/research/2026-07-17-retail-external-container-looting-pseudocode.md

@@ -45,6 +45,12 @@ Cross-references:
 
 The strip is LayoutDesc `0x21000008`, selected root `0x10000063`
 (`0,400`, `800 x 110`) rather than a synthetic inventory window.
+The root's own DirectState is only the tiled center surface `0x06004CC2`; the
+common floating-window nine-slice supplies the missing outer bevel. The authored
+Left/Right edge rules stretch the two horizontal lists and scrollbar while
+pinning the close button, so the mounted strip resizes horizontally only. The
+modern retained mount starts at a compact 700-pixel content width and may be
+resized from either side up to the viewport edge.
 
 | Element | Purpose | Authored bounds |
 |---|---|---|
@@ -67,6 +73,12 @@ UseWorldObject(objectId):
         expectedExternalRoot = objectId
     send Use(objectId)
 
+UseCorpse(corpseId):
+    // A corpse is a TYPE_CONTAINER carrying BF_STUCK | BF_CORPSE.
+    // BF_STUCK excludes DetermineUseResult's PlaceInBackpack branch.
+    classify through ItemHolder::UseObject
+    send Use(corpseId), never PutItemInContainer(corpseId)
+
 OnViewContents(containerId, orderedEntries):
     // Always update the projection: ACE sends nested-container snapshots too.
     objectTable.ReplaceContents(containerId, orderedEntries)
@@ -132,9 +144,21 @@ DragExternalItemToOwnedInventory(itemId, placement, splitAmount):
         send StackableSplitToContainer(itemId, player/openOwnedBag,
                                        placement, splitAmount)
     else:
+        destinationList.InsertItem(itemId, placement)
+        destinationList.pendingItem.SetWaitingState(true)
         send PutItemInContainer(itemId, player/openOwnedBag, placement)
     do not mutate canonical ownership before the server response
 
+OnServerMoveItem(itemId):
+    if destinationList.pendingItem.itemId == itemId:
+        clear pendingItem
+        rebuild the list from authoritative placement
+
+OnInventoryServerSaveFailed(itemId):
+    if destinationList.pendingItem.itemId == itemId:
+        remove the pending copy
+        leave the source ownership unchanged
+
 DragOwnedItemToExternalContainer(itemId, placement, splitAmount):
     if splitAmount < full stack:
         send StackableSplitToContainer(itemId, currentExternalSubcontainer,
@@ -157,6 +181,10 @@ DragOwnedItemToExternalContainer(itemId, placement, splitAmount):
   projection owner.
 - `ExternalContainerController` owns the authored strip, range/close behavior,
   nested-list selection, selection, and drag/drop presentation.
+- `InventoryController` owns retail `UIElement_ItemList::m_pendingItem`: a
+  destination-only waiting projection at the exact chosen slot. It subscribes
+  to authoritative move and failure notices and never rewrites external-item
+  ownership while the request is in flight.
 - `ItemInteractionController` remains the only interpreter of retail item-use
   and placement policy. It gains explicit adapters for `SetGroundObject` and
   `PlaceInContainer`; panels do not reimplement those decisions.