Replace the single-column `<fk table='...' column='...'/>` marker with a
unified, composite-capable format `<fk table='...' columns='local:foreign,...'/>`
and emit full column lists for many-to-many markers, so clients can
reconstruct composite relationships.
- Match every real table relationship a column participates in, instead of
only single-column foreign keys, still preferring tables over the
view-derived relationships.
- Emit the full ordered local->foreign column mapping in the `<fk>` marker
and the full source/target column lists in the `<m2m>` marker.
- Add composite foreign key and composite m2m fixtures plus OpenAPI spec
assertions.
jimmy
72b9a116be