Cacheless single-file API: WriteDocument encodes the declared type, the registry is the call's argument, DecodeDocument moves to CultCodec, ReadStoreSnapshot's gate is pinned

SerializePayload takes the declared type. A call attaches its registry under the store's gate and restores the previous one, so no call leaves a registry on the store. Not yet run; the next commit set is verified on Yggdrasil.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs

@@ -802,9 +802,9 @@ public class SingleFileMessagePackBackingStore : SingleFileBackingStore
         return store;
     }
 
-    protected override byte[] SerializePayload(object document)
+    protected override byte[] SerializePayload(object document, Type documentType)
     {
-        return CultDocumentMessagePackSerialization.SerializeUntyped(document, document.GetType(), Registry);
+        return CultDocumentMessagePackSerialization.SerializeUntyped(document, documentType, Registry);
     }
 
     protected override object DeserializePayload(Type documentType, byte[] payload)