Watch graceful shutdown - #923

Rebased! Been running locally with success.

Tested locally on Node 24 (Windows). All 6 new graceful shutdown unit tests in test/watch-unit.test.js pass cleanly and npm run lint passes with 0 warnings.

A few points verified in the diff:

1. Tracking liveChildren in a Set ensures children undergoing restart transitions are still sent GRACEFUL_SHUT and drained rather than orphaned when SIGINT or SIGTERM arrives.
2. Passing closeGraceDelay into lib/watch/fork.js unifies the timeout between the supervisor and child instead of hardcoding 5000ms.
3. Setting allStop = true and checking stopping guards against re-entrant shutdown from repeated signals or late file system watch events.
4. Exiting with 130 on SIGINT and 143 on SIGTERM matches standard process convention.