ADR-0087 exemption (no-migration-prescription): This narrows a REST route's runtime read, not a metadata surface: no Zod schema, spec declaration or stored representation changes here, and `objectstack migrate meta` has nothing to rewrite for it. The one at-rest spelling with a measured population is `reference_to`, already carried by the pre-existing ADR-0087 entry `field-reference-to-alias`, which is untouched by this change and disjoint from the three spellings it removes; those three have no at-rest population and are refused by name at the write door, so a new ledger entry would be scope invented at conversion time.