ADR-0087 exemption (no-migration-prescription): No authorable key is renamed, retired or re-typed: `source` keeps its name and meaning, and every envelope that carried a non-blank `source` parses byte-identically. The two newly refused spellings — an envelope carrying only `ast`, and a `source` that is blank after trimming — never evaluated on any release (the `ast`-only one faulted at run time with the engine's own "persist `source`" prescription, the blank one with a parse error), and a repo-wide census found no in-repo instance of either, so `objectstack migrate meta` has nothing to rewrite and the remedy is authoring a `source`, which the refusal itself prescribes.