feat(items): port retail external-container looting

Add the ClientUISystem ground-object lifecycle, authoritative root and nested ViewContents projections, replacement and close semantics, and the DAT-authored gmExternalContainerUI strip for chests and corpses.

Route double-click loot and full or partial drag transfers through the shared retail item policy without optimistic external ownership. Remove the incorrect NoLongerViewingContents behavior from owned side packs and retire AP-106/#196.

Release build succeeds and all 5,875 tests pass with five intentional skips.

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

src/AcDream.App/World/ExternalContainerLifecycleController.cs

@@ -0,0 +1,51 @@
+using AcDream.Core.Items;
+
+namespace AcDream.App.World;
+
+/// <summary>
+/// Owns the network side effect of replacing retail's current ground object.
+/// Presentation controllers observe the same state but do not own session
+/// lifecycle messages.
+/// </summary>
+public sealed class ExternalContainerLifecycleController : IDisposable
+{
+    private readonly ExternalContainerState _state;
+    private readonly ClientObjectTable _objects;
+    private readonly Action<uint> _sendNoLongerViewingContents;
+    private bool _disposed;
+
+    public ExternalContainerLifecycleController(
+        ExternalContainerState state,
+        ClientObjectTable objects,
+        Action<uint> sendNoLongerViewingContents)
+    {
+        _state = state ?? throw new ArgumentNullException(nameof(state));
+        _objects = objects ?? throw new ArgumentNullException(nameof(objects));
+        _sendNoLongerViewingContents = sendNoLongerViewingContents
+            ?? throw new ArgumentNullException(nameof(sendNoLongerViewingContents));
+        _state.Changed += OnChanged;
+    }
+
+    private void OnChanged(ExternalContainerTransition transition)
+    {
+        // ClientUISystem::SetGroundObject @ 0x00564510 always queues the old
+        // root's contents tree for destruction when the ground object changes.
+        if (transition.PreviousContainerId != 0u)
+            _objects.StopViewingContentsTree(transition.PreviousContainerId);
+
+        // Only replacement emits Event_NoLongerViewingContents. The authored
+        // close/range path instead sends Use(root) and waits for event 0x0052.
+        if (transition.Kind == ExternalContainerTransitionKind.Replaced
+            && transition.PreviousContainerId != 0u)
+        {
+            _sendNoLongerViewingContents(transition.PreviousContainerId);
+        }
+    }
+
+    public void Dispose()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        _state.Changed -= OnChanged;
+    }
+}