feat(engine): GREEN — acceptance engine, state-shaped writers, ApplyWrites transport (task 4) master
Deterministic digest rkeys (golden-vector pinned); state-shaped acceptance/removal/restore writers (PDS has no upsert/tolerant-delete — pre-read + shape + swapCommit guard); CID-stable re-fires; engine ProcessAdmission routing with deferred-on-undecided discipline; admit.go policy core split from ledger reservation (task-3 tests untouched-and-green). Side-effect production fixes: 400+RecordNotFound now maps to ErrNotFound (retired the self-retiring pinned-defect test — retried deletes were 500ing), InvalidSwap now maps to ErrSwapConflict (comments' concurrent-edit detection was dead code against a real PDS). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>