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).

scripts/build-rootfs.sh

@@ -40,7 +40,6 @@ esac
 EDIR="$(edition_dir "$EDITION")"
 ROOTFS="$ROOTFS_DIR/$EDITION"
 ARTIFACT="$ARTIFACT_DIR/arcline-$EDITION-$VERSION-$ARCH.tar.xz"
-LIVE="${ARCLINE_LIVE:-0}"
 
 mkdir -p "$ROOTFS_DIR" "$LOG_DIR" "$ARTIFACT_DIR"
 rm -rf "$ROOTFS" "$ARTIFACT"
@@ -135,7 +134,7 @@ case "$ARCLINE_TOOLCHAIN" in
 esac
 
 mount_pseudo
-chroot_run "ARCLINE_EDITION_DIR='/root/arcline-edition' ARCLINE_LIVE='$LIVE' ARCLINE_LOCK_ROOT='${ARCLINE_LOCK_ROOT:-0}' ARCLINE_EXTRA_REPOS='${ARCLINE_EXTRA_REPOS:-0}' /root/configure-system.sh '$EDITION'"
+chroot_run "ARCLINE_EDITION_DIR='/root/arcline-edition' ARCLINE_LOCK_ROOT='${ARCLINE_LOCK_ROOT:-0}' ARCLINE_EXTRA_REPOS='${ARCLINE_EXTRA_REPOS:-0}' /root/configure-system.sh '$EDITION'"
 rm -f "$ROOTFS/root/configure-system.sh"
 rm -rf "$ROOTFS/root/arcline-edition" "$ROOTFS/arcline-debs"
 unmount_pseudo