Refuse a backup destination an unplugged volume left empty (closes #239)

Every backup writes vaultik-destination.txt at the destination root.
While the local index holds snapshots stored there, a destination
without it is not present: snapshot create refuses before writing,
prune and snapshot purge fail, and snapshot list and snapshot remove
act as for a destination that cannot be listed. snapshot create and
snapshot remove check it first. remote nuke deletes the marker last.
The file:// backend never creates its destination directory, which
protects a directory on a removable volume, not a mount point.

Tests back up into directories that exist; fixtures for a written
destination carry the marker.

Judgement call: the marker is rewritten on every backup.
Deviation: for the complexity limit, two blocks moved, unchanged, into
snapshotNamesToCreate and confirmRemoveSnapshot.

Model: opus-5-5

internal/vaultik/deep_verify_test.go

@@ -35,7 +35,7 @@ func TestDeepVerifyAcceptsHealthyAndRejectsCorruptBlob(t *testing.T) {
 	tempDir := t.TempDir()
 
 	dataDir := filepath.Join(tempDir, "source")
-	storeDir := filepath.Join(tempDir, "remote")
+	storeDir := t.TempDir()
 	dbPath := filepath.Join(tempDir, "index.sqlite")
 
 	chunkSize := int64(64 * 1024)
@@ -94,7 +94,7 @@ func TestDeepVerifyJSONCountsBlobs(t *testing.T) {
 
 	fs := afero.NewOsFs()
 	ctx := context.Background()
-	storeDir := filepath.Join(t.TempDir(), "remote")
+	storeDir := t.TempDir()
 
 	cfg, storer, snapshotID, keys := backUpSeveralBlobs(ctx, t, storeDir)
 
@@ -151,7 +151,7 @@ func TestDeepVerifyJSONCountsWrongSizeBlob(t *testing.T) {
 
 	fs := afero.NewOsFs()
 	ctx := context.Background()
-	storeDir := filepath.Join(t.TempDir(), "remote")
+	storeDir := t.TempDir()
 
 	cfg, storer, snapshotID, keys := backUpSeveralBlobs(ctx, t, storeDir)
 
@@ -183,7 +183,7 @@ func TestDeepVerifyStopsOnBlobThatCannotBeChecked(t *testing.T) {
 	t.Parallel()
 
 	ctx := context.Background()
-	storeDir := filepath.Join(t.TempDir(), "remote")
+	storeDir := t.TempDir()
 
 	cfg, storer, snapshotID, keys := backUpSeveralBlobs(ctx, t, storeDir)
 	failing := statFailingStorer{Storer: storer, failKeys: []string{keys[0]}}