feat(ui): D.2b empty-slot art — ItemListCellTemplate resolver (0x1000000e -> 0x21000037)

Ports UIElement_ItemList::InternalCreateItem (0x004e3570) attribute lookup to a
testable static helper. The 0x1000000e property is an EnumBaseProperty (not
DataIdBaseProperty as the spec anticipated) — discovered by runtime reflection and
confirmed against the live dat. Pinned sprite ids from real-dat test:
  contents  (0x100001C6): 0x06004D20
  side-bag  (0x100001CA): 0x06005D9C
  main-pack (0x100001C9): 0x06005D9C

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.App.Tests/UI/Layout/ItemListCellTemplateTests.cs

@@ -0,0 +1,59 @@
+using System;
+using System.IO;
+using AcDream.App.UI.Layout;
+using DatReaderWriter;
+using DatReaderWriter.Options;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace AcDream.App.Tests.UI.Layout;
+
+/// <summary>
+/// Real-dat test for <see cref="ItemListCellTemplate.ResolveEmptySprite"/>: the inventory
+/// item-lists must resolve their empty-slot sprite from the dat cell template (attribute
+/// 0x1000000e -> catalog 0x21000037 prototype's ItemSlot_Empty), NOT the generic toolbar
+/// square 0x060074CF. Also PRINTS the resolved ids — the "pin the exact asset" record.
+/// Skips when the live dat directory is absent (CI).
+/// </summary>
+public class ItemListCellTemplateTests
+{
+    private const uint Generic = 0x060074CFu;   // generic toolbar-shortcut empty square (the bug)
+
+    private const uint ContentsLayout = 0x21000021u; private const uint ContentsGrid = 0x100001C6u;
+    private const uint BackpackLayout = 0x21000022u; private const uint SideBag = 0x100001CAu;
+    private const uint MainPack = 0x100001C9u;
+
+    private readonly ITestOutputHelper _out;
+    public ItemListCellTemplateTests(ITestOutputHelper o) => _out = o;
+
+    private static string? DatDir()
+    {
+        var d = Environment.GetEnvironmentVariable("ACDREAM_DAT_DIR")
+            ?? Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile),
+                            "Documents", "Asheron's Call");
+        return Directory.Exists(d) ? d : null;
+    }
+
+    [Fact]
+    public void Inventory_lists_resolve_a_real_nongeneric_empty_sprite()
+    {
+        var datDir = DatDir();
+        if (datDir is null) return;   // CI: no live dat — skip (smoke test)
+
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+
+        foreach (var (name, layout, elem) in new[]
+        {
+            ("contents",  ContentsLayout, ContentsGrid),
+            ("side-bag",  BackpackLayout, SideBag),
+            ("main-pack", BackpackLayout, MainPack),
+        })
+        {
+            uint sprite = ItemListCellTemplate.ResolveEmptySprite(dats, layout, elem);
+            _out.WriteLine($"{name}: list 0x{elem:X8} (layout 0x{layout:X8}) -> empty sprite 0x{sprite:X8}");
+            Assert.True(sprite != 0u,       $"{name}: resolved 0 (no 0x1000000e attr / prototype / media)");
+            Assert.True(sprite != Generic,  $"{name}: resolved the generic 0x060074CF — the bug, not the fix");
+            Assert.True((sprite & 0xFF000000u) == 0x06000000u, $"{name}: 0x{sprite:X8} is not a 0x06 RenderSurface id");
+        }
+    }
+}