The idempotency contract is the marker's scope, so both directions are
pinned: an unchanged set must reuse its marker (or a 15-minute cron
repeats itself forever) and a changed one must not (or a misparse hides
under a marker the thread already carries). crew#308's negated clause is
replayed through the sweep, and the empty parse is echoed beside the
untouched `blocked-unparseable` flag.
Refs #252
