fix(os): fix FEDAC-PIECE permission denied — chmod 777 partition + write test
The ext4 partition root is owned by root after mkfs, so liveuser/kioskuser
can't write kiosk.log. chmod 777 at build time, and add a write test
before committing to the log path so the session falls back to /tmp
gracefully if permissions are still wrong.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>