ADR-0087 exemption (no-migration-prescription): A validity narrowing over an existing key: `reference` is not removed, renamed or re-shaped, so there is no tombstone and nothing mechanical for `objectstack migrate meta` to rewrite. The parse refusal is the channel that reaches an affected author, at the parse site, carrying the remedy; which target object a blank `lookup` / `master_detail` was meant to point at is authoring intent no migration entry can decide on an upgrader's behalf — and the measured population of affected sources is zero across all tracked files (census on the PR). Mirrors the disposition of #13632, whose emptiness test this completes.