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

docs/DATAMODEL.md

@@ -183,7 +183,7 @@ Tracks blob upload metrics.
    - `INSERT OR REPLACE INTO files` - Update file metadata
    - `SELECT * FROM files WHERE path = ?` - Check if file has changed
    - `INSERT INTO snapshot_files` - Add file to current snapshot
-   - `INSERT INTO snapshot_mount_points` - Record each directory on a different filesystem from its parent
+   - `INSERT OR IGNORE INTO snapshot_mount_points` - Record each directory on a different filesystem from its parent
 
 2. **Chunking** (for changed files)
    - `INSERT OR IGNORE INTO chunks` - Store new chunks

internal/snapshot/device_number_test.go

@@ -10,9 +10,9 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-// The device number a real backup reads is the same for two entries of
-// one directory and differs between two filesystems. Without it no
-// mount point is ever found. See
+// The device number a real backup reads is the same for a directory and
+// a file in it, and differs between the temp directory and /dev.
+// Without it no mount point is ever found. See
 // https://git.eeqj.de/sneak/vaultik/issues/395.
 func TestDeviceNumberFromStat(t *testing.T) {
 	t.Parallel()