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>

tests/unit/backup/test_compose_hard_restart.py

@@ -0,0 +1,65 @@
+from __future__ import annotations
+
+import unittest
+from typing import List
+from unittest.mock import patch
+
+
+class HardRestartArgTests(unittest.TestCase):
+    """The hard-restart list defaults to empty (no compose down/up); callers
+    opt in per dir, e.g. compose hosts pass 'mailu' while swarm hosts, where
+    the dir is a stack whose overlay network collides with compose up, pass
+    nothing."""
+
+    def _parse(self, extra: List[str]):
+        import sys
+
+        from baudolo.backup import cli
+
+        argv = [
+            "baudolo",
+            "--compose-dir",
+            "/tmp",
+            "--backups-dir",
+            "/tmp/backup",
+            "--database-containers",
+            "postgres",
+            "--images-no-stop-required",
+            "redis",
+            *extra,
+        ]
+        with patch.object(sys, "argv", argv):
+            return cli.parse_args()
+
+    def test_default_is_empty(self) -> None:
+        args = self._parse([])
+        self.assertEqual(args.hard_restart_projects, [])
+
+    def test_empty_flag_stays_empty(self) -> None:
+        args = self._parse(["--hard-restart-projects"])
+        self.assertEqual(args.hard_restart_projects, [])
+
+    def test_explicit_names_preserved(self) -> None:
+        args = self._parse(["--hard-restart-projects", "mailu", "foo"])
+        self.assertEqual(args.hard_restart_projects, ["mailu", "foo"])
+
+    def test_backups_dir_is_required(self) -> None:
+        import sys
+
+        from baudolo.backup import cli
+
+        argv = [
+            "baudolo",
+            "--compose-dir",
+            "/tmp",
+            "--database-containers",
+            "postgres",
+            "--images-no-stop-required",
+            "redis",
+        ]
+        with patch.object(sys, "argv", argv), self.assertRaises(SystemExit):
+            cli.parse_args()
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)