Handle atomic-save file replacements

[onlylisten]

Summary

• handle atomic-save workflows that replace the file with rename
• re-establish the watcher after file replacement or watcher errors
• clear pending debounce and rewatch timers when rotating watchers

Why

ColaMD's live reload currently relies on fs.watch, but many editors and AI tools save by writing a temp file and renaming it into place on macOS. That emits rename instead of change, which leaves the old watcher attached to the replaced inode and breaks subsequent real-time updates.

Testing

• npm run build