SPEC.airc phase2-dynamic 노드 일부. registry-client.fl 신규 — GET /watch/:service long-poll + 재연결 + jitter 50~500ms(dec-009) + 5s polling fallback. plain HTTP(dec-007). state: {:registry-url :service :since :running}. API: rc-create / rc-start! / rc-stop! / rc-on-change(callback). check: register 후 3초 이내 callback 호출, watch 끊김 시뮬레이션 → polling fallback → 복구 후 long-poll.