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>

tests/AcDream.Core.Tests/Items/ClientObjectTableTests.cs

@@ -46,6 +46,41 @@ public sealed class ClientObjectTableTests
         Assert.Equal(1, propUpdateCount);
     }
 
+    [Fact]
+    public void StopViewingContents_RemovesOnlyTemporaryProjection()
+    {
+        var repo = new ClientObjectTable();
+        ClientObject item = MakeItem(100u);
+        item.ContainerId = 77u;
+        repo.AddOrUpdate(item);
+        repo.ReplaceContents(77u, new uint[] { 100u });
+        uint? changed = null;
+        repo.ContainerContentsReplaced += id => changed = id;
+
+        Assert.True(repo.StopViewingContents(77u));
+
+        Assert.Empty(repo.GetContents(77u));
+        Assert.Same(item, repo.Get(100u));
+        Assert.Equal(77u, item.ContainerId);
+        Assert.Equal(77u, changed);
+        Assert.False(repo.StopViewingContents(77u));
+    }
+
+    [Fact]
+    public void StopViewingContentsTree_RemovesNestedSnapshotsButPreservesObjects()
+    {
+        var repo = new ClientObjectTable();
+        repo.ReplaceContents(77u, new[] { new ContainerContentEntry(88u, 1u) });
+        repo.ReplaceContents(88u, new[] { new ContainerContentEntry(100u, 0u) });
+
+        Assert.Equal(2, repo.StopViewingContentsTree(77u));
+
+        Assert.Empty(repo.GetContents(77u));
+        Assert.Empty(repo.GetContents(88u));
+        Assert.NotNull(repo.Get(88u));
+        Assert.NotNull(repo.Get(100u));
+    }
+
     [Fact]
     public void MoveItem_UpdatesContainerAndFiresEvent()
     {