Machine-readable data dictionary: per-dataset point-in-time semantics (which timestamp is first-seen vs data vintage, w…

Machine-readable data dictionary: per-dataset point-in-time semantics (which timestamp is first-seen vs data vintage, what mutates vs what is frozen, how corrections work), field documentation, refresh cadences, and the endpoints/MCP tools serving each dataset. The DDQ answer sheet for data-engineering reviewers.