GAP-ER-3: first-class versioned DATASET_VERSION entity (ModelCarryManifest has model, not dataset, version)

[mdheller]

Surfaced by AgentCognitionER (ER-1, prophet-workspace#115 / matrix prophet-workspace#116).

ModelCarryManifest carries a first-class MODEL_VERSION (entries[].modelRef + contentSha256 + manifest version), but a DATASET_VERSION exists only as provenance (sourceKind/sha256RequiredBeforeEligibility) + examples/embedding-carry-ref.json — not a versioned, chainable entity.

The ER tooth requires every PROVENANCE_RECORD depends_on DATASET_VERSION (with a MODEL_VERSION). Today that edge can't be uniformly enforced.

Ask: a versioned DatasetVersion (id + contentSha256 + optional prevSha256 chain) so provenance can name its data as first-class as its model. ER verdict today: PARTIAL.