Follow-up to the salvaged #93517. Two gaps found in review:
- After the last unregister the supervisor stayed resident for the life of
the process (~15 MB + a pipe) in any gateway/cron that ever connected a
stdio server; main's per-server watchdog exited with its server. Close
our write end when the supervised set empties: EOF with nothing
registered makes the supervisor exit without reaping, and the next
register already respawns and replays.
- A child that raced and exited before os.getpgid dropped its group from
coverage entirely. The SDK spawns stdio servers as session leaders
(pgid == pid), so fall back to the pid instead; the prune forgets the
group once nothing in it is alive.
Tests: fake-supervisor release/re-spawn sequence, and a real-process EOF
release (supervisor exits 0, unregistered child untouched). Mutation
checked: disabling the release branch fails both.
4a2b23d77e