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/README.md

@@ -0,0 +1,38 @@
+# btrfs in Arcline OS
+
+Arcline OS is **btrfs-native**: the root filesystem layout is created at
+install time and snapshots are first-class, not bolted on.
+
+## Subvolume layout
+
+| Subvolume    | Mount point   | Purpose                                        |
+|--------------|---------------|------------------------------------------------|
+| `@`          | `/`           | system root — rolled back on failed upgrade    |
+| `@home`      | `/home`       | user data — not rolled back by default         |
+| `@log`       | `/var/log`    | logs survive rollbacks                         |
+| `@snapshots` | `/.snapshots` | read-only snapshots live here                  |
+
+The layout is created by `btrfs/init.sh <device>` (this is what the installer
+runs). `/etc/fstab` templates live in each edition manifest and reference these
+subvolumes with `subvol=` mount options.
+
+## Tooling
+
+- `btrfs/init.sh <device>` — create the subvolume layout on a target device.
+- `btrfs/snapshot.sh` — snapshot / list / prune. Installed in images as
+  `/usr/local/sbin/arcline-snapshot` and triggered automatically by the
+  `arcline-snapshot.timer` systemd unit (see `overlays/base/usr/lib/systemd/system/`).
+- `btrfs/rollback.sh` — promote a snapshot back to `@`. Installed as
+  `/usr/local/sbin/arcline-rollback`.
+
+## How a rollback works
+
+Rollbacks are done from a rescue/live environment (never against the mounted
+`@`). The tool moves the current `@` aside to `@.rollback-<ts>` and promotes
+the chosen snapshot to `@`. Because btrfs renames are atomic, a crash mid-way
+leaves a recoverable system on disk.
+
+## Compression
+
+All subvolumes mount with `compress=zstd:3` (see the edition `fstab`
+templates). Transparent compression is on by default.