feat: add btrfs subvolume, snapshot, and rollback tooling

- init.sh: creates the @ / @home / @log / @snapshots subvolume layout
  on a target device (the installer step).
- snapshot.sh: scheduled read-only snapshots with pruning, installed as
  /usr/local/sbin/arcline-snapshot.
- rollback.sh: safe boot-to-snapshot rollback that refuses to touch the
  live @ and promotes a snapshot atomically.

btrfs/init.sh

@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# ─────────────────────────────────────────────────────────────────────────────
+# Arcline OS — btrfs subvolume layout initialiser
+#
+#   btrfs/init.sh <device>            (e.g. /dev/sda2)
+#   btrfs/init.sh --list <device>
+#
+# Creates the Arcline subvolume layout on a fresh (or existing) btrfs device:
+#
+#     @            → /            system root (rolled back on failure)
+#     @home        → /home        user data
+#     @log         → /var/log     logs (not rolled back, survives)
+#     @snapshots   → /.snapshots  where arcline-snapshot keeps snapshots
+#
+# This is the step the installer runs; it is safe to re-run (existing
+# subvolumes are kept, missing ones are created). Requires root.
+# ─────────────────────────────────────────────────────────────────────────────
+set -euo pipefail
+
+DEV="${1:?usage: btrfs/init.sh <device>}"
+TOP="/run/arcline-btrfs-init"
+
+require_root() {
+  if [[ $EUID -ne 0 ]] && command -v sudo >/dev/null; then
+    exec sudo -E "$0" "$@"
+  elif [[ $EUID -ne 0 ]]; then
+    echo "error: needs root" >&2; exit 1
+  fi
+}
+require_root "$0" "$@"
+
+command -v btrfs >/dev/null || { echo "error: btrfs-progs not installed" >&2; exit 1; }
+
+if ! blkid -p -O "$DEV" | grep -q btrfs 2>/dev/null; then
+  echo "error: $DEV is not a btrfs filesystem" >&2
+  exit 1
+fi
+
+mkdir -p "$TOP"
+mount -o subvolid=5 "$DEV" "$TOP"
+trap 'umount "$TOP" 2>/dev/null || true; rmdir "$TOP" 2>/dev/null || true' EXIT
+
+create_subvol() {
+  local name="$1"
+  if [[ -d "$TOP/$name" ]]; then
+    echo "  keeping existing subvolume: $name"
+  else
+    echo "  creating subvolume:        $name"
+    btrfs subvolume create "$TOP/$name" >/dev/null
+  fi
+}
+
+echo "Arcline btrfs layout on $DEV:"
+create_subvol "@"
+create_subvol "@home"
+create_subvol "@log"
+create_subvol "@snapshots"
+
+# mark @snapshots read-only friendly (snapper/arcline-snapshot manage it)
+btrfs property set -ts "$TOP/@snapshots" ro false 2>/dev/null || true
+
+echo "done. The installer mounts:"
+echo "  @          → /"
+echo "  @home      → /home"
+echo "  @log       → /var/log"
+echo "  @snapshots → /.snapshots"