feat(archsetup): back up system files before in-place edits

Add a backup_system_file helper that snapshots a pre-existing file to <path>.archsetup.bak before archsetup edits it in place, so a botched edit to fstab, mkinitcpio.conf, or sudoers is recoverable. It is idempotent: it never overwrites an existing backup, so the pristine original survives repeated edits within a run and across re-runs. It uses cp -p to preserve mode and ownership.

Only the in-place sed and append edits to pre-existing files route through it (locale.gen, makepkg.conf, pacman.conf, sudoers, wireless-regdom, geoclue.conf, pacman-contrib, fstab, mkinitcpio.conf, vconsole.conf). The brand-new drop-in files archsetup fully owns are skipped: there is no prior state to save, and recovery is just deleting them.

Covered by tests/backup-system-file/ (Normal, Boundary, Error cases, including mode preservation and the no-overwrite guarantee).

0 files changed, 0 insertions, 0 deletions