feat(render #53): EntityClassificationCache.Populate + roundtrip tests

Implements Populate (insert-or-overwrite) and adds 5 tests covering the
populate->TryGet round-trip including the Setup pre-flatten shape. Per
spec test plan section 7.1 tests #2, #3, #9, #10, #14.

Tests use xUnit Assert.* (not FluentAssertions) to match the Task 2
implementer's choice and the existing 149 sibling assertions in the Wb
test directory.

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

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

@@ -17,6 +17,92 @@ public class EntityClassificationCacheTests
         Assert.Null(entry);
     }
 
+    [Fact]
+    public void Populate_ThenTryGet_ReturnsBatchesInOrder()
+    {
+        var cache = new EntityClassificationCache();
+        var batches = new[]
+        {
+            MakeCachedBatch(ibo: 1, firstIndex: 0, indexCount: 6, texHandle: 0xAA),
+            MakeCachedBatch(ibo: 1, firstIndex: 6, indexCount: 6, texHandle: 0xBB),
+        };
+
+        cache.Populate(entityId: 100, landblockHint: 0xA9B40000u, batches);
+
+        Assert.True(cache.TryGet(100, out var entry));
+        Assert.NotNull(entry);
+        Assert.Equal(100u, entry!.EntityId);
+        Assert.Equal(0xA9B40000u, entry.LandblockHint);
+        Assert.Equal(batches, entry.Batches);
+    }
+
+    [Fact]
+    public void Populate_OverridesExistingEntry()
+    {
+        var cache = new EntityClassificationCache();
+        cache.Populate(100, 0u, new[] { MakeCachedBatch(1, 0, 6, 0xAA) });
+        cache.Populate(100, 0u, new[] { MakeCachedBatch(2, 0, 12, 0xCC) });
+
+        Assert.True(cache.TryGet(100, out var entry));
+        Assert.NotNull(entry);
+        Assert.Single(entry!.Batches);
+        Assert.Equal(0xCCu, entry.Batches[0].BindlessTextureHandle);
+    }
+
+    [Fact]
+    public void Count_TracksLiveEntries()
+    {
+        var cache = new EntityClassificationCache();
+        Assert.Equal(0, cache.Count);
+
+        cache.Populate(1, 0u, new[] { MakeCachedBatch(1, 0, 6, 0xAA) });
+        Assert.Equal(1, cache.Count);
+
+        cache.Populate(2, 0u, new[] { MakeCachedBatch(2, 0, 6, 0xAA) });
+        Assert.Equal(2, cache.Count);
+
+        // Re-populate same id — should not double-count.
+        cache.Populate(1, 0u, new[] { MakeCachedBatch(3, 0, 6, 0xBB) });
+        Assert.Equal(2, cache.Count);
+    }
+
+    [Fact]
+    public void Populate_WithEmptyBatches_StoresEmptyEntry()
+    {
+        var cache = new EntityClassificationCache();
+        cache.Populate(entityId: 7, landblockHint: 0u, System.Array.Empty<CachedBatch>());
+
+        Assert.True(cache.TryGet(7, out var entry));
+        Assert.NotNull(entry);
+        Assert.Empty(entry!.Batches);
+    }
+
+    [Fact]
+    public void Populate_SetupMultiPart_StoresFlatBatchPerSubPart()
+    {
+        // Synthetic Setup with 3 subParts × 2 batches each = 6 flat entries.
+        // This pins the spec §3 Q4 decision: pre-flatten Setup multi-parts at
+        // populate time so the per-frame hot path is branchless.
+        var cache = new EntityClassificationCache();
+        var batches = new CachedBatch[6];
+        for (int subPart = 0; subPart < 3; subPart++)
+        for (int b = 0; b < 2; b++)
+        {
+            batches[subPart * 2 + b] = MakeCachedBatch(
+                ibo: (uint)(subPart + 1),
+                firstIndex: (uint)(b * 6),
+                indexCount: 6,
+                texHandle: (ulong)(0x100 + subPart * 2 + b));
+        }
+        cache.Populate(99, 0u, batches);
+
+        Assert.True(cache.TryGet(99, out var entry));
+        Assert.NotNull(entry);
+        Assert.Equal(6, entry!.Batches.Length);
+        Assert.Equal(0x100u, entry.Batches[0].BindlessTextureHandle);
+        Assert.Equal(0x105u, entry.Batches[5].BindlessTextureHandle);
+    }
+
     private static CachedBatch MakeCachedBatch(
         uint ibo, uint firstIndex, int indexCount, ulong texHandle)
     {