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.Content/IPreparedAssetSource.cs

@@ -131,7 +131,10 @@ internal static class PreparedAssetRequestContract
 {
     public static void ValidateType(PakAssetType type)
     {
-        if (!Enum.IsDefined(type))
+        if (type is not (
+            PakAssetType.GfxObjMesh or
+            PakAssetType.SetupMesh or
+            PakAssetType.EnvCellMesh))
         {
             throw new ArgumentOutOfRangeException(
                 nameof(type),
@@ -166,7 +169,9 @@ internal static class PreparedAssetRequestContract
 /// Production prepared source backed by one immutable memory-mapped pak.
 /// There is intentionally no DAT fallback.
 /// </summary>
-public sealed class PakPreparedAssetSource : IPreparedAssetSource
+public sealed partial class PakPreparedAssetSource :
+    IPreparedAssetSource,
+    IPreparedCollisionSource
 {
     private readonly PakReader _reader;
     private readonly Action<string>? _diagnosticSink;