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/user/systemd/user/kanshi.service

@@ -0,0 +1,17 @@
+# ABOUTME: systemd user service for kanshi dynamic display configuration.
+# ABOUTME: Starts kanshi as part of the graphical session, with automatic restart.
+
+[Unit]
+Description=Dynamic display configuration
+Documentation=man:kanshi(1)
+PartOf=graphical-session.target
+After=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/kanshi
+Restart=on-failure
+RestartSec=3
+
+[Install]
+WantedBy=graphical-session.target