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/ExternalContainerStateTests.cs

@@ -0,0 +1,70 @@
+using AcDream.Core.Items;
+
+namespace AcDream.Core.Tests.Items;
+
+public sealed class ExternalContainerStateTests
+{
+    [Fact]
+    public void ViewContents_onlyCommitsTheExpectedRoot()
+    {
+        var state = new ExternalContainerState();
+        state.RequestOpen(0x70000001u);
+
+        Assert.False(state.ApplyViewContents(0x70000002u));
+        Assert.Equal(0u, state.CurrentContainerId);
+        Assert.True(state.ApplyViewContents(0x70000001u));
+        Assert.Equal(0x70000001u, state.CurrentContainerId);
+    }
+
+    [Fact]
+    public void ReplacementPublishesOldAndNewExactlyOnce()
+    {
+        var state = new ExternalContainerState();
+        var changes = new List<ExternalContainerTransition>();
+        state.Changed += changes.Add;
+        state.RequestOpen(1u);
+        state.ApplyViewContents(1u);
+        state.RequestOpen(2u);
+        state.ApplyViewContents(2u);
+        state.ApplyViewContents(2u);
+
+        Assert.Equal(2, changes.Count);
+        Assert.Equal(ExternalContainerTransitionKind.Opened, changes[0].Kind);
+        Assert.Equal(
+            new ExternalContainerTransition(
+                ExternalContainerTransitionKind.Replaced, 1u, 2u),
+            changes[1]);
+    }
+
+    [Fact]
+    public void CloseIgnoresStaleContainerAndClosesCurrent()
+    {
+        var state = Open(2u);
+
+        Assert.False(state.ApplyClose(1u));
+        Assert.Equal(2u, state.CurrentContainerId);
+        Assert.True(state.ApplyClose(2u));
+        Assert.Equal(0u, state.CurrentContainerId);
+        Assert.Equal(0u, state.RequestedContainerId);
+    }
+
+    [Fact]
+    public void RefusedReplacementRestoresCurrentAsExpectedRoot()
+    {
+        var state = Open(1u);
+        state.RequestOpen(2u);
+
+        Assert.True(state.ApplyUseDone(0x550u));
+        Assert.Equal(1u, state.CurrentContainerId);
+        Assert.Equal(1u, state.RequestedContainerId);
+        Assert.False(state.ApplyViewContents(2u));
+    }
+
+    private static ExternalContainerState Open(uint id)
+    {
+        var state = new ExternalContainerState();
+        state.RequestOpen(id);
+        state.ApplyViewContents(id);
+        return state;
+    }
+}