RepositoryExportRow
Type: warmhub.RepositoryExportRow
RepositoryExportRow = JsonMappingOne validated export row.
Deliberately a mapping rather than a dataclass per row kind: data is the
user’s own Shape fields 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.