metadata.remove

Remove an anchored-metadata entry. Strips the anchor content-control wrapper (its content stays in the document) and deletes the payload entry from the Storage Part. In v1 these writes are sequenced, not transactional: the adapter resolves the target up-front so missing-target failures land before any state change, but a crash strictly between the two writes can leave a dangling payload. When the backing part has no remaining entries, the part itself is removed.