mart-author

Author a new medallion node (gold mart, silver dim, or additive column) when the live gold layer AND the content pack both cannot serve a business need. Takes the user's business logic, inspects the Fusion PVO SOURCE schema (not bronze) for available raw fields, and authors the lowest-cost, additive, non-destructive change as content-pack YAML + SQL in an overlay pack — never touching already-materialized (possibly terabyte-scale) bronze/silver. Then validates and hands off to seed. Use when oac-dataset-advisor reports a true GAP, or the user says 'add a metric/dimension my gold layer doesn't have', 'create a new mart for <business logic>', 'I need a column that doesn't exist'. Does NOT seed, query live data, alter existing nodes' grain/keys, or write Python dim modules.