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.Mesh/CultMeshSingleFileDocuments.cs

@@ -112,7 +112,7 @@ namespace GameCult.Mesh
             var descriptor = resolvedRegistry.GetRequired<TDocument>();
             var payload = ReadSingleFileDocumentPayload(path, key, descriptor.SchemaId);
             // A load refuses what a write refuses, and mints the ids a cache load of the same file would mint.
-            return StoreAt(path).DecodeDocument<TDocument>(key, payload, resolvedRegistry);
+            return CultCacheMessagePack.CreateCodec(resolvedRegistry).DecodeDocument<TDocument>(key, payload);
         }
 
         /// <summary>