Implements all three changes from the knowledge-core-completeness tech spec:
1. Named graph field preserved through Cassandra storage (7-element tuple),
enabling provenance triples to retain their graph URIs on round-trip.
2. Provenance triples already arrive on triples-input — no routing change
needed; Change 1 was sufficient.
3. Source material (library documents) streamed alongside triples and
embeddings during core download/upload. The knowledge manager fetches
the document hierarchy from the librarian on download and recreates it
on upload, preserving the full provenance chain across instances.
