feat(backup): capture volumes from a filesystem snapshot

Backing up a live volume with rsync copies a moving target: a database
written to mid-copy lands on disk in a state no engine ever committed.
Stopping the container avoids that at the cost of downtime.

A snapshot removes both. `--snapshot {btrfs,zfs}` with `--snapshot-subject`
freezes the docker root once per run, and every volume copy is then read
from that frozen tree while the containers keep serving. A restore of such
a copy is an ordinary crash recovery, which every supported engine performs
on its own at startup.

An unsupported filesystem or an unknown snapshot kind fails loudly rather
than degrading to a live copy, since a silent fallback would return exactly
the torn backup the mode exists to prevent. `--shutdown` is rejected
alongside `--snapshot` instead of being ignored: under a snapshot no
container is ever stopped, so accepting the flag would promise downtime
semantics the run does not deliver.

Copies out of a snapshot skip rsync's --checksum verification. The source
is immutable for the lifetime of the copy, so size-and-mtime cannot race,
and dropping the second full read roughly halves the I/O per volume.

backup/app.py grew past what one module could carry and is split into
layout, policy and dumps along the lines it already had internally.

Tests: unit coverage for the new snapshot, layout, policy, volume and cli
units; e2e cases drive real btrfs, zfs and ext4 filesystems on loop devices
in a privileged container, including a MariaDB that is written to across
the snapshot and must recover from the restored copy without losing a
committed row. CI installs zfs and sets E2E_REQUIRE_FILESYSTEMS so a
missing kernel module fails the build instead of silently skipping a
filesystem.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/baudolo/backup/layout.py

@@ -0,0 +1,33 @@
+"""Where a backup run puts its files, and how a finished run is stamped."""
+
+from __future__ import annotations
+
+import os
+import pathlib
+
+from dirval import create_stamp_file
+
+from .shell import execute_shell_command
+
+
+def get_machine_id() -> str:
+    return execute_shell_command("sha256sum /etc/machine-id")[0][0:64]
+
+
+def stamp_directory(version_dir: str) -> None:
+    """
+    Use dirval as a Python library to stamp the directory (no CLI dependency).
+    """
+    create_stamp_file(version_dir)
+
+
+def create_version_directory(versions_dir: str, backup_time: str) -> str:
+    version_dir = os.path.join(versions_dir, backup_time)
+    pathlib.Path(version_dir).mkdir(parents=True, exist_ok=True)
+    return version_dir
+
+
+def create_volume_directory(version_dir: str, volume_name: str) -> str:
+    path = os.path.join(version_dir, volume_name)
+    pathlib.Path(path).mkdir(parents=True, exist_ok=True)
+    return path