Use pymacos instead of pync for the nc alerter

[JeanExtreme002]

The nc alerter relies on pync, which hasn't had a release since 2.0.3 (May 2018). pync works by shelling out to the terminal-notifier app it bundles, and that binary is Intel-only:

$ file pync/vendor/terminal-notifier-2.0.0/terminal-notifier.app/Contents/MacOS/terminal-notifier
Mach-O 64-bit executable x86_64

On Apple silicon it only runs under Rosetta. Apple has said Rosetta stays available as a general-purpose tool only through macOS 27, so the alerter has a deadline. Also, when notifications are turned off, pync's alerts vanish without any error.

This switches the alerter to pymacos, a pure-Python library with no dependencies:

• pync.notify(message=..., title="SimpleMonitor") becomes macos.notify(message, title="SimpleMonitor"). It posts through AppleScript, with the text passed as arguments, so quotes and emoji in monitor names need no escaping.
• If notifications are turned off for the app that posts them (Script Editor), pymacos raises instead of macOS dropping the alert. Following the other alerters, the failure is logged with alerter_logger.exception("Couldn't send notification") and doesn't stop the run.
• pyproject.toml: pync is replaced by pymacos = {version = "^1.19", platform = "darwin"}. poetry.lock was regenerated with the same Poetry version (2.4.1), so the diff is just that swap and the content hash.
• docs/alerters/nc.rst: notes the Script Editor attribution and the logged error.
• tests/test_nc.py: checks that a failure is notified with the right title, that a notification error is logged and not raised, and that dry_run sends nothing. These tests fail against the current pync code.

Disclosure: I'm the author of pymacos.

Checked locally:

• On macOS (Apple silicon), pytest tests passes (189 tests), and ruff check, ruff format --check and mypy are clean.
• I ran the real alerter with a real MonitorFail named disk "backup" café 🚀, with a stub osascript on PATH so no real notification was posted. pymacos called it with the message Monitor disk "backup" café 🚀 on <host> failed and the title SimpleMonitor, unchanged.