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/validate.sh

@@ -21,8 +21,15 @@ while IFS= read -r -d '' s; do
   if ! bash -n "$s"; then
     note_fail "syntax error in $s"
   fi
-done < <(find "$ROOT/scripts" "$ROOT/btrfs" "$ROOT/toolchain" "$ROOT/tests" \
-           -name '*.sh' -type f -print0 2>/dev/null)
+done < <(find "$ROOT/scripts" "$ROOT/btrfs" "$ROOT/toolchain" "$ROOT/tests" "$ROOT/overlays" \
+           -type f \( -name '*.sh' -o -name 'arcline-installer-session' \) -print0 2>/dev/null)
+
+log "validating installer (python syntax)…"
+if command -v python3 >/dev/null 2>&1; then
+  python3 -m py_compile "$ROOT/installer/arcline-installer" 2>/dev/null \
+    || note_fail "python syntax error in installer/arcline-installer"
+  rm -rf "$ROOT/installer/__pycache__"
+fi
 
 log "validating editions…"
 for e in "${EDITIONS[@]}"; do