[harshilv17]

Closes #569

Adds an --on-watch option (FASTIFY_ON_WATCH env, or onWatch in a config file) to start --watch. On every watched change the command runs through the shell with its output piped through, and the app reloads after the command exits:

fastify start -w --on-watch 'npm run lint' app.js

• A non-zero exit is logged (on-watch command exited with code N) and the app still reloads, so a lint error doesn't stop the watcher.
• Changes made while the command is running don't start another run; the reload that follows picks them up. A burst of saves runs the command once.
• Files the command writes should go in --ignore-watch, otherwise each run triggers the next one. The help text and README say so.
• After the watcher is stopped, a command that finishes doesn't fork a new child.

Without --on-watch the watcher behaves as before.

I also checked it by hand on a small app: two touch app.js ran the command twice and reloaded the server twice (same restart count as plain --watch), and --on-watch 'exit 3' logged the exit code and kept serving.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[SameRainbows]

Tested locally on Node 24. The unit tests in test/watch-unit.test.js cover command execution before restart, debouncing during an active run, non-zero exit logging, and suppression of restarts after stop.

One edge case to consider: if emitter.stop() is called while an on-watch command is still executing, stop() terminates the running application children in childs but does not terminate the spawned child process. Holding a reference to the active spawn process and calling .kill() on it inside emitter.stop() would ensure long-running commands (such as a slow build or type check) do not linger after the watcher halts.

[harshilv17]

@SameRainbows Good catch, thanks. Pushed a fix: stop() now kills a running on-watch command, and the test checks it.