DATA_SPACE_SOURCE_CODEC: NodeSpecCodec<DataSpaceSource> = ...

A data space source (PLAN ยง11.25): its data space, execution context and entity, the query builder's lambda and its relation, each as the engine's JSON, all three left out while it is unset, and once resolved its schema snapshot, so inference works offline. Unresolved and failed sources have no snapshot and load unresolved: the host resolves them again.