feat(content): bake and read flat collision assets

Append strict collision/topology payloads to the existing prepared package so later physics cutover can drop parsed DAT graphs without adding a second mapping or changing traversal behavior. The full 2,232,170-key catalog is deterministic across worker counts, exact-byte aliased, corruption-isolated, and cancellation-safe.

src/AcDream.Bake/BakeArtifactValidator.cs

@@ -8,7 +8,8 @@ public static class BakeArtifactValidator
     public static void Validate(
         string path,
         in PakHeader expectedHeader,
-        int expectedTocCount)
+        int expectedTocCount,
+        IReadOnlyDictionary<PakAssetType, int>? expectedTypeCounts = null)
     {
         using var reader = new PakReader(path);
         var actual = reader.Header;
@@ -35,5 +36,18 @@ public static class BakeArtifactValidator
                 $"{expectedTocCount}");
 
         reader.ValidateTocStructure();
+        if (expectedTypeCounts is null)
+            return;
+
+        foreach ((PakAssetType type, int expectedCount) in expectedTypeCounts)
+        {
+            int actualCount = reader.CountEntries(type);
+            if (actualCount != expectedCount)
+            {
+                throw new InvalidDataException(
+                    $"bake catalog type {type} contains {actualCount} keys; " +
+                    $"expected {expectedCount}");
+            }
+        }
     }
 }