feat: graphical installer in the live ISO

The ISO now boots straight into a GTK installer instead of dropping to a
tty. Structure:

- installer/arcline-installer: small GTK3 (Python) frontend that drives
  scripts/deploy-disk.sh — pick a disk, choose boot mode, type the device
  path to confirm, watch the deploy log, reboot. Pure helper logic is
  tested against lsblk (lowercase keys, pseudo-devices filtered).
- scripts/build-live.sh: builds build/rootfs/<edition>-live by cloning the
  CLEAN rootfs and layering on live-boot, a minimal X session (Xorg +
  openbox), the installer, and the deploy tooling under /usr/lib/arcline
  (deploy-disk.sh + btrfs/init.sh + edition fstabs, laid out so the
  scripts' own path resolution works unchanged).
- overlays/live/: arcline-installer.service + session script that start
  Xorg on vt1 (with -allow-root) and run the installer as the X client.
- build-iso.sh: builds the live rootfs for the squashfs AND stages the
  clean rootfs archive into isofiles/install/ — the installer deploys the
  clean archive, so what's installed is the hardened system, never the
  live session with the installer in it.
- Refactor: ARCLINE_LIVE handling removed from build-rootfs.sh and
  configure-system.sh (now lives entirely in build-live.sh).
- validate.sh now checks overlays shell scripts + installer python.
- docs updated (building.md, architecture.md, installer/README.md).

overlays/live/etc/systemd/system/arcline-installer.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Arcline graphical installer (live session)
+Documentation=docs/building.md
+After=systemd-user-sessions.service systemd-logind.service
+Conflicts=getty@tty1.service
+
+[Service]
+Type=simple
+ExecStart=/usr/local/sbin/arcline-installer-session
+StandardInput=tty
+TTYPath=/dev/tty1
+TTYReset=yes
+Restart=on-failure
+
+[Install]
+WantedBy=multi-user.target