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.App.Tests/UI/UiItemListTests.cs

@@ -114,4 +114,36 @@ public class UiItemListTests
         list.ScrollItemIntoView(0);
         Assert.Equal(0, list.Scroll.ScrollY);
     }
+
+    [Fact]
+    public void HorizontalScroll_UsesPixelOffsetAndKeepsPartialEdgeCells()
+    {
+        var list = new UiItemList
+        {
+            Width = 80f,
+            Height = 32f,
+            CellWidth = 32f,
+            CellHeight = 32f,
+            SingleRow = true,
+            HorizontalScroll = true,
+        };
+        list.Flush();
+        for (int i = 0; i < 4; i++) list.AddItem(new UiItemSlot());
+
+        list.Scroll.SetScrollY(17);
+        list.LayoutCells();
+
+        Assert.Equal(128, list.Scroll.ContentHeight);
+        Assert.Equal(80, list.Scroll.ViewHeight);
+        Assert.Equal(-17f, list.GetItem(0)!.Left);
+        Assert.True(list.GetItem(0)!.Visible);
+        Assert.True(list.GetItem(3)!.Visible);
+        Assert.False(list.GetItem(2) is null);
+
+        list.ScrollItemIntoView(3);
+        Assert.Equal(48, list.Scroll.ScrollY);
+        list.LayoutCells();
+        Assert.Equal(48f, list.GetItem(3)!.Left);
+        Assert.Equal(80f, list.GetItem(3)!.Left + list.GetItem(3)!.Width);
+    }
 }