Extract shared helpers into lib.sh (log, err, confirm, path constants)
and refactor post-install.sh + update.sh to source it.
New transform.sh converts an existing Arch+Wayland system to Moonarch:
pre-flight summary, config backup, DM conflict resolution, PulseAudio
removal, full package install, and hard overwrite of all configs.
Also migrate kanshi from niri spawn-at-startup to systemd user service.
