DR-050 — owner_kind is the grain discriminator; content_type is never overloaded as a grain key

Result-grain identity in corpus search (and any polymorphic multi-grain surface) is carried by a dedicated owner_kind value (DR-036 lineage — the record_embeddings vocabulary); content_type remains the closed editorial taxonomy on source_documents only. A new grain extends the owner_kind vocabulary rather than misclassifying under an existing kind. Grounded in ontology CVs 04-content-type + 22-origin-kind (neither is a grain key); resolves the bl-431 OBS-2 sd-arm conflation (Task 144, OD-2).