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 holds an entry and is on a different device from
its parent directory, looked up once the walk has listed it, as listing
mounts an automounted volume. Under --prune, when an earlier snapshot
of that name recorded such a directory and this run does not, the run
skips the prune, keeps every older snapshot and exits non-zero, naming
the directory. For a volume retired on purpose, snapshot purge removes
the older snapshots.

Model: opus-5-5

TODO.md

@@ -24,6 +24,14 @@ owner's; a `v*` tag runs the release build in
 
 # Completed Steps
 
+- 2026-10-15: Stopped `snapshot create --prune` pruning after a backup
+  in which a volume mounted below a configured path in an earlier
+  snapshot of the same name was not mounted
+  ([issue #395](https://git.eeqj.de/sneak/vaultik/issues/395)). Such a
+  prune could delete every snapshot still holding the volume's files.
+  Each snapshot now records in the local index the directories it found
+  on a different filesystem from their parent, with something in them.
+
 - 2026-10-14: Stopped `snapshot create --prune` pruning after a backup
   that found nothing in a configured path
   ([issue #388](https://git.eeqj.de/sneak/vaultik/issues/388)), as in