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 checks
before dropping incomplete snapshots, snapshot remove before looking on
the destination. 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
sneak