Make nested snapshots opt-in; fix snapshot leaks found in audit

Opt-in
------
Nested-dataset snapshot support changes how backups read their source data, so
it is now off by default and gated behind a marker file:

  install.sh --enable-nested-snapshots
  install.sh --disable-nested-snapshots

With neither flag install.sh preserves the current setting, so a routine
`git pull && bash install.sh` can never silently flip it. When disabled,
apply.sh skips the patch entirely and the stock guard remains. uninstall.sh
tears down staging mounts and removes the marker.

Snapshot lifecycle
------------------
zfs.snapshot.delete defaults to recursive=False and stock restic_backup() calls
it with no options. Stock is safe only because its validation means recursive is
never True in the field. Enabling nested datasets makes recursive snapshots real:
the parent then has one child snapshot per descendant dataset (160+ on an Apps
pool), so stock's delete would orphan every child on EVERY successful run.

The patch now owns the lifecycle end to end:

- delete_snapshot_tree() sweeps the parent and all children, and is idempotent
  against stock's finally winning the race once our mounts are released
- on a staging failure the tree is deleted here, because sync.py never completes
  `snapshot, local_path = await create_snapshot(...)` and so its finally deletes
  nothing at all
- the snapshot is recorded in a sidecar file before anything is mounted, so a
  middlewared restart mid-backup cannot orphan it
- a crashed run's snapshot tree is reclaimed on the next run instead of being
  overwritten and leaked

Silent-omission fix
-------------------
The dataset list is now enumerated AFTER the snapshot. Read beforehand it could
miss a dataset created in the gap, which the recursive snapshot would capture but
the staging plan would not -- silently omitting its data. Read afterwards, an
unsnapshotted dataset trips the staging check and fails the run loudly.

Also from the audit
-------------------
- plan_staging scopes by dataset name, so skipped-dataset warnings no longer
  include every mountpoint-less dataset on the box, which buried the ones that
  matter
- staging_root_for rejects "." / ".." components that would escape the staging
  base, and resolves STAGING_BASE at call time rather than freezing it into a
  default argument
- uninstall.sh no longer `rm -rf`s a tree that may still contain live bind
  mounts, and unmounts by path depth rather than string length
- apply_plan takes an injectable isdir; verify_staged drops an unused parameter
- pin the shellcheck action instead of tracking @master

61 tests, ruff and shellcheck clean.

.gitignore

@@ -4,6 +4,7 @@
 /apply.log.2
 /hook_status.json
 /disabled
+/nested_snapshots_enabled
 
 # Python
 __pycache__/