AssetManager: drop immediate SaveGUIDRegistry on registration

2268fa5 added the dirty-flag deferred flush but left the immediate
save in place, so new GUID registrations wrote the registry twice
(once inline, once at the next FlushGUIDRegistry). Remove the inline
save so the deferred path is the only writer.

Engine/src/Assets/AssetManager.cpp

@@ -49,8 +49,7 @@ namespace Engine::Assets {
 
         m_PathToGuid.emplace(path, guid);
         m_GuidToPath.emplace(guid, path);
-        SaveGUIDRegistry();
-        m_GuidRegistryDirty = true;
+        m_GuidRegistryDirty = true;   // 延迟落盘：由 FlushGUIDRegistry 统一写
         return guid;
     }