Read-only ActivityPub → atproto bridge for the threadiverse using Coves lexicons

wip(task16): repo.ApplyOpsTx side-effect (both branches), internal/acceptrec leaf extraction, engine create/crash/opt-out master

acceptrec lifts the acceptance CAS/removal-guard from materialize (goldens preserved); Create{Page} enqueue rides the acceptance commit tx (atomic); opt-out check moved into the engine to record rejections. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>