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>

src/AcDream.App/Rendering/Wb/EntityClassificationCache.cs

@@ -48,4 +48,18 @@ internal sealed class EntityClassificationCache
     /// </summary>
     public bool TryGet(uint entityId, out EntityCacheEntry? entry)
         => _entries.TryGetValue(entityId, out entry);
+
+    /// <summary>
+    /// Insert or overwrite a cache entry for <paramref name="entityId"/>.
+    /// Defensive: if an entry already exists, replaces it.
+    /// </summary>
+    public void Populate(uint entityId, uint landblockHint, CachedBatch[] batches)
+    {
+        _entries[entityId] = new EntityCacheEntry
+        {
+            EntityId = entityId,
+            LandblockHint = landblockHint,
+            Batches = batches,
+        };
+    }
 }