fix: verify_rollback sentinel must live on the rolled-back dataset

/root is mounted on a separate dataset (zroot/home/root, created by
archangel:create_datasets), but verify_rollback was snapshotting
zroot/ROOT/default. The rollback was a no-op for the sentinel file,
so the post-rollback existence check failed — the visible symptom
was a PASSED test with a soft-failure warning
("Rollback failed - test file not restored" →
"Rollback verification had issues") that persisted across ZFS
configs for weeks.

Move the sentinel to /etc/archangel-rollback-test. /etc has no child
dataset mounted there, so the file lives on zroot/ROOT/default —
the dataset actually being snapshotted and rolled back.

Defensively single-quote $test_file at the five ssh_cmd call-sites
so future path changes (whitespace, special chars) stay correct
without touching each call again.

The 2026-04-21 VM run logged "Rollback verified - test file restored"
on zfs-mirror-encrypt, confirming the fix.

0 files changed, 0 insertions, 0 deletions