fix: pid reuse after reboot, missing pid in instance.json, Windows non-ASCII locale

[nicoloboschi]

Fixes #37, fixes #36, fixes #35.

#37: saved pid reused after reboot

Every running check was kill -0 <saved pid>. After a reboot, macOS gave that pid to another process, so start refused forever. Worse, stop/drop could fall back to kill -9 on that unrelated process.

• is_process_running is now is_postgres_process: the pid must be alive and its executable must be postgres (via ps -o comm= on Unix, QueryFullProcessImageNameW on Windows). pid 0 is rejected.
• New running_pid(&info): the saved pid must also match <data_dir>/postmaster.pid. stop, drop, psql, install-extension and the health check use it.
• start checks the pid in postmaster.pid rather than the saved one, and never removes the pidfile of a live postgres.
• SIGKILL fallbacks only fire after re-checking that the pid is still postgres.

#36: instance.json without pid

InstanceInfo.pid is now Option<u32>, so a missing pid or null loads. 0 is treated as not running.

#35: Windows non-ASCII locale

postgresql_embedded gives no way to pass --locale to initdb. On Windows, when the data dir isn't initialized yet, pg0 now runs initdb itself with the crate's exact arguments plus --locale=C, and setup() then skips its own initdb. Existing clusters and Linux/macOS behavior are unchanged.

Testing

Unit tests (cargo test, now run in the macOS CI job; the suite wasn't run in CI before):

• is_postgres_process: a live fake postgres counts; a live non-postgres pid (reused), pid 0, and an exited process don't.
• running_pid: a matching live postgres counts; a missing/0 pid, a reused pid, a pid that doesn't match postmaster.pid, and a missing pidfile don't.
• check_not_running (start's pre-check): refuses a live postmaster and keeps its pidfile; removes a reused-pid, garbage, or absent pidfile.
• kill_if_postgres: leaves a non-postgres process alive; kills postgres.
• wait_for_shutdown: not done while postgres is alive; done when the pid is reused and the pidfile is gone.
• Health check: false for a reused pid and for no pid.
• instance.json: missing pid, null pid, round trip.
• init_data_dir: runs the real bundled initdb; asserts lc_* = C, password auth, temp password file removed.

Manual run on macOS: start → stop → remove pid → start ✅. pid=1 in both instance.json and postmaster.pid: info says not running, stop signals nothing, start recovers ✅. Double start still refuses ✅.

Windows: the Windows build and Windows SDK tests passed on the first commit, so QueryFullProcessImageNameW compiles and new clusters get initialized through the new initdb path.