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/configure-system.sh

@@ -17,7 +17,6 @@ EDITION="${1:?usage: configure-system.sh <edition>}"
 DISTRO_NAME="${DISTRO_NAME:-Arcline OS}"
 VERSION="${VERSION:-0.1.0}"
 RELEASE_NAME="${RELEASE_NAME:-arclines}"
-LIVE="${ARCLINE_LIVE:-0}"
 LOCK_ROOT="${ARCLINE_LOCK_ROOT:-0}"
 EXTRA_REPOS="${ARCLINE_EXTRA_REPOS:-0}"
 
@@ -85,13 +84,6 @@ fi
 # ensure no package telemetry survives.
 rm -f /var/log/apt/*.log /var/cache/apt/archives/*.deb
 
-# ── live-boot (ISO builds) ──────────────────────────────────────────────────
-if [[ "$LIVE" == "1" ]]; then
-  log "installing live-boot support"
-  export DEBIAN_FRONTEND=noninteractive
-  apt-get install -y --no-install-recommends live-boot live-config-systemd live-tools || warn "live-boot install failed"
-fi
-
 # ── Arcline toolchain (host-built .debs) ────────────────────────────────────
 if [[ -d /arcline-debs ]] && ls /arcline-debs/*.deb >/dev/null 2>&1; then
   log "installing Arcline toolchain packages"