Atomically applies an array of CompositionOps. All ops succeed or none do (transactional). Pass `if_match` for optimistic-concurrency control (409 stale_etag on mismatch). Pass `dry_run: true` to validate the batch — runs the same apply + projection accept/reject guards WITHOUT persisting, returning `{ valid, errors }` with `locator.op_index` on the first failing op.