Add transform script for existing Arch+Wayland systems

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.

defaults/xdg/niri/config.kdl

@@ -77,7 +77,7 @@ layout {
 }
 
 // xwayland-satellite is managed automatically since niri 25.08
-spawn-at-startup "kanshi"
+// kanshi is managed via systemd user service (kanshi.service)
 spawn-at-startup "waybar"
 spawn-at-startup "dunst"
 spawn-at-startup "/usr/lib/polkit-gnome/polkit-gnome-authentication-agent-1"