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/snapshot/module.go

@@ -1,6 +1,8 @@
 package snapshot
 
 import (
+	"os"
+
 	"github.com/spf13/afero"
 	"go.uber.org/fx"
 	"sneak.berlin/go/vaultik/internal/config"
@@ -19,6 +21,9 @@ type ScannerParams struct {
 	// PreviousSnapshotID is the most recent completed snapshot of the
 	// same name, "" for none.
 	PreviousSnapshotID string
+	// DeviceNumber is ScannerConfig.DeviceNumber; nil reads it from the
+	// file's stat.
+	DeviceNumber func(info os.FileInfo) (uint64, bool)
 }
 
 // Module exports backup functionality as an fx module.
@@ -59,6 +64,7 @@ func provideScannerFactory(
 			Exclude:            excludes,
 			SkipErrors:         params.SkipErrors,
 			PreviousSnapshotID: params.PreviousSnapshotID,
+			DeviceNumber:       params.DeviceNumber,
 		})
 	}
 }