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, 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. remote nuke deletes it last. The
file:// backend no longer creates its destination directory and checks
for it on every write; that protects a directory on a removable
volume, not a mount point.

Tests back up into directories that exist; fixtures standing for a
written destination carry the marker; two tests check they get past
it.

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/restore_sweeper_test.go

@@ -34,7 +34,7 @@ func TestRestoreFreesBlobsNoPendingFileNeeds(t *testing.T) {
 	dataDir := filepath.Join(tempDir, "source")
 	originalsDir := filepath.Join(dataDir, "originals")
 	copiesDir := filepath.Join(dataDir, "copies")
-	storeDir := filepath.Join(tempDir, "remote")
+	storeDir := t.TempDir()
 	dbPath := filepath.Join(tempDir, "index.sqlite")
 
 	originals := writeFilesWithCopies(t, fs, originalsDir, copiesDir, fileBytes)