Do not prune when a backup misses an earlier mount point (closes #395)

snapshot create --prune purged the older snapshots holding a volume's
files when the volume, mounted below a configured path, was not
mounted: its empty mount point read as deleted files.

The scan now records in the local index each directory at or below a
configured path that is on a different device from its parent
directory. Under --prune, when an earlier snapshot of that name
recorded such a directory and this run does not find it on a different
device, the run skips the prune, keeps every older snapshot, names the
directory and exits non-zero, as for an empty configured path. For a
volume retired, or a directory no longer backed up, on purpose, the
older snapshots are removed with snapshot purge.

Model: opus-5-5

internal/vaultik/snapshot_prune_test.go

@@ -325,7 +325,8 @@ func TestBackupWithVolumeNotMountedDoesNotPrune(t *testing.T) {
 		err := env.v.CreateSnapshot(prune)
 		require.ErrorIs(t, err, vaultik.ErrVolumeNotMounted)
 		assert.Contains(t, env.out.String(),
-			"did not find a mount point at "+volumeDir+" of snapshot "+snapshotName)
+			"did not find a mount point at "+volumeDir+" of snapshot "+
+				snapshotName)
 		assert.Contains(t, env.out.String(), "'vaultik snapshot purge'")
 		assert.NotContains(t, env.out.String(), "Running post-backup prune.")
 	}