feat(render #53): EntityClassificationCache skeleton + first test

Adds CachedBatch, EntityCacheEntry, and EntityClassificationCache with
just TryGet (returns false on empty). The skeleton compiles and the first
test (TryGet_EmptyCache_ReturnsFalse) passes. Subsequent tasks add
Populate, InvalidateEntity, InvalidateLandblock, and the dispatcher
integration. Per spec design Section 6.1.

Note: CachedBatch / EntityCacheEntry / EntityClassificationCache are
internal (not public as the plan snippet showed). Their members
transitively reference the internal GroupKey type, so promoting them to
public produces CS0051 inconsistent-accessibility errors. The cache is
dispatcher-internal coordination state anyway, and the AcDream.App
csproj already exposes internals to AcDream.Core.Tests via
InternalsVisibleTo, so the test sees everything it needs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/AcDream.Core.Tests/Rendering/Wb/EntityClassificationCacheTests.cs

@@ -0,0 +1,33 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.App.Rendering.Wb;
+using AcDream.Core.Meshing;
+using Xunit;
+
+namespace AcDream.Core.Tests.Rendering.Wb;
+
+public class EntityClassificationCacheTests
+{
+    [Fact]
+    public void TryGet_EmptyCache_ReturnsFalse()
+    {
+        var cache = new EntityClassificationCache();
+        bool found = cache.TryGet(entityId: 42, out var entry);
+        Assert.False(found);
+        Assert.Null(entry);
+    }
+
+    private static CachedBatch MakeCachedBatch(
+        uint ibo, uint firstIndex, int indexCount, ulong texHandle)
+    {
+        var key = new GroupKey(
+            Ibo: ibo,
+            FirstIndex: firstIndex,
+            BaseVertex: 0,
+            IndexCount: indexCount,
+            BindlessTextureHandle: texHandle,
+            TextureLayer: 0,
+            Translucency: TranslucencyKind.Opaque);
+        return new CachedBatch(key, texHandle, Matrix4x4.Identity);
+    }
+}