fix(items): finish corpse pickup and combat switching

Port retail's first-slot ShowPendingInPlayer path for double-click loot and carry the current owned-container destination through deferred pickup. Retire the previous ground-container view as soon as a replacement is requested so its range close cannot cancel ACE's active MoveTo chain.

Preserve active-combat weapon intent across ACE's authoritative wand-to-missile stance tail while leaving peace-mode switches unchanged.

Release build succeeds and all 5,885 tests pass with five intentional skips.

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

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

@@ -22,6 +22,10 @@ Named Sept-2013 retail symbols:
 - `UIElement_ItemList::ListenToElementMessage @ 0x004E4D50`
 - `ClientUISystem::SetGroundObject @ 0x00564510`
 - `ClientUISystem::OnViewContents @ 0x00565870`
+- `CPlayerSystem::PlaceInBackpack @ 0x0055D8C0`
+- `gmInventoryUI::RecvNotice_ShowPendingInPlayer @ 0x004A6B50`
+- `gmInventoryUI::RecvNotice_EndPendingInPlayer @ 0x004A68B0`
+- `UIElement_ItemList::ItemList_InsertItem @ 0x004E3F40`
 - `ACCWeenieObject::UIAttemptPutInContainer @ 0x0058D680`
 - `ACCWeenieObject::UIAttemptSplitToContainer @ 0x0058D7D0`
 - `ItemHolder::AttemptToPlaceInContainer @ 0x00588140`
@@ -69,9 +73,22 @@ thumb, and arrow sprites from LayoutDesc `0x2100003E`.
 ```text
 UseWorldObject(objectId):
     decision = ItemHolder policy
-    if decision is SetGroundObject:
-        expectedExternalRoot = objectId
     send Use(objectId)
+    if decision is SetGroundObject:
+        SetGroundObject(objectId, notifyServer = true)
+
+SetGroundObject(newRoot, notifyServer):
+    if currentExternalRoot == newRoot:
+        return
+    oldRoot = currentExternalRoot
+    if oldRoot != 0:
+        retire oldRoot's temporary contents tree
+        publish SetGroundObject(0) to the external panel
+        if notifyServer:
+            send NoLongerViewingContents(oldRoot)
+    currentExternalRoot = newRoot
+    expectedExternalRoot = newRoot
+    // The panel remains empty until ViewContents for newRoot is accepted.
 
 UseCorpse(corpseId):
     // A corpse is a TYPE_CONTAINER carrying BF_STUCK | BF_CORPSE.
@@ -86,11 +103,6 @@ OnViewContents(containerId, orderedEntries):
     if containerId != expectedExternalRoot:
         return
 
-    old = currentExternalRoot
-    if old != 0 and old != containerId:
-        objectTable.StopViewingContentsTree(old)
-        send NoLongerViewingContents(old) exactly once
-
     currentExternalRoot = containerId
     expectedExternalRoot = containerId
     externalPanel.SetGroundObject(containerId)
@@ -138,6 +150,14 @@ OnExternalItemDoubleClick(itemId):
     ItemHolder.UseObject(itemId)
     // Because item.ContainerId is currentExternalRoot, retail policy emits
     // PlaceInBackpack; the server remains authoritative for the move.
+    destination = current open owned pack
+    destinationList.RemoveTrailingEmptyItem()
+    destinationList.AddEmptySlot(at index 0)
+    destinationList.AddItem(itemId)
+    destinationList.pendingItem.SetWaitingState(true)
+    send PutItemInContainer(itemId, destination, placement = 0)
+    // The pending item therefore reserves the FIRST slot and shifts the
+    // existing packed list to the right until the server confirms or rejects.
 
 DragExternalItemToOwnedInventory(itemId, placement, splitAmount):
     if splitAmount < full stack:
@@ -171,8 +191,10 @@ DragOwnedItemToExternalContainer(itemId, placement, splitAmount):
 ## Architectural mapping
 
 - `ExternalContainerState` in Core owns only expected/current external-root
-  identity and transition ordering. It filters ACE's nested `ViewContents`
-  packets so they cannot replace the visible root.
+  identity and transition ordering. A replacement request retires current
+  presentation immediately, matching `SetGroundObject`; accepted
+  `ViewContents` later opens the requested root. It filters ACE's nested
+  `ViewContents` packets so they cannot replace the visible root.
 - `ExternalContainerLifecycleController` in App observes those transitions,
   retires the replaced root's temporary preview tree, then owns the
   replacement-only `NoLongerViewingContents` network side effect. The wire