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.Core/Items/ExternalContainerState.cs

@@ -0,0 +1,98 @@
+using System;
+
+namespace AcDream.Core.Items;
+
+public enum ExternalContainerTransitionKind
+{
+    Opened,
+    Replaced,
+    Closed,
+    Reset,
+}
+
+public readonly record struct ExternalContainerTransition(
+    ExternalContainerTransitionKind Kind,
+    uint PreviousContainerId,
+    uint ContainerId);
+
+/// <summary>
+/// Owns retail <c>ClientUISystem::groundObject</c> and the outstanding external
+/// container request. ACE sends ViewContents for the requested root and then for
+/// nested containers; only the expected root may become the visible ground object.
+/// Retail: <c>ClientUISystem::OnViewContents @ 0x00565870</c> and
+/// <c>SetGroundObject @ 0x00564510</c>.
+/// </summary>
+public sealed class ExternalContainerState
+{
+    public uint RequestedContainerId { get; private set; }
+    public uint CurrentContainerId { get; private set; }
+
+    public event Action<ExternalContainerTransition>? Changed;
+
+    public bool RequestOpen(uint containerId)
+    {
+        if (containerId == 0u || RequestedContainerId == containerId)
+            return false;
+        RequestedContainerId = containerId;
+        return true;
+    }
+
+    public bool ApplyViewContents(uint containerId)
+    {
+        if (containerId == 0u || containerId != RequestedContainerId)
+            return false;
+
+        uint previous = CurrentContainerId;
+        CurrentContainerId = containerId;
+        RequestedContainerId = containerId;
+        if (previous == containerId)
+            return false;
+
+        Changed?.Invoke(new ExternalContainerTransition(
+            previous == 0u
+                ? ExternalContainerTransitionKind.Opened
+                : ExternalContainerTransitionKind.Replaced,
+            previous,
+            containerId));
+        return true;
+    }
+
+    public bool ApplyClose(uint containerId)
+    {
+        if (containerId == 0u || containerId != CurrentContainerId)
+            return false;
+
+        uint previous = CurrentContainerId;
+        CurrentContainerId = 0u;
+        RequestedContainerId = 0u;
+        Changed?.Invoke(new ExternalContainerTransition(
+            ExternalContainerTransitionKind.Closed,
+            previous,
+            0u));
+        return true;
+    }
+
+    public bool ApplyUseDone(uint weenieError)
+    {
+        if (weenieError == 0u || RequestedContainerId == CurrentContainerId)
+            return false;
+        RequestedContainerId = CurrentContainerId;
+        return true;
+    }
+
+    public bool Reset()
+    {
+        uint previous = CurrentContainerId;
+        bool changed = previous != 0u || RequestedContainerId != 0u;
+        CurrentContainerId = 0u;
+        RequestedContainerId = 0u;
+        if (changed)
+        {
+            Changed?.Invoke(new ExternalContainerTransition(
+                ExternalContainerTransitionKind.Reset,
+                previous,
+                0u));
+        }
+        return changed;
+    }
+}