ADR-0087 exemption (no-migration-prescription): A runtime accept-set narrowing on the engine's predicate-update path: no authorable metadata key is removed, renamed or re-shaped, so there is no tombstone and nothing for `objectstack migrate meta` to rewrite. The affected artifact is HOOK BODY CODE, whose per-row intent no mechanical rewrite can infer — choosing between a `ctx.api` per-row write and by-id updates is an authoring decision. The refusal itself is the notification channel, raised at the write site with the object, the diverging keys and both routes in the envelope.