RepositoryExportRow
Type: warmhub.RepositoryExportRow
RepositoryExportRow = JsonMappingOne validated export row.
Deliberately a mapping rather than a dataclass per row kind: data holds the
fields of the row’s certified Shape set and the row’s optional members differ
by kind, so a frozen four-class hierarchy would buy type names at the cost of a
decode the caller then has to undo. The envelope keys — kind, wref,
durableId, version, active — are guaranteed present by validation.