Gate r1 (2ab + 2c, Medium): the tick regression stubbed _process_due_job and
held only recurring jobs, so neither the `except OSError` around
claim_job_for_fire nor the "one-shots still dispatch past a failed advance"
filter was exercised; reverting either kept every test green.
Add a due one-shot to the same test, stub run_one_job instead, and assert the
one-shot's execution row closes as failed ("Cron store unwritable") rather
than being left claimed. Red with the claim guard reverted (row stays
"claimed") and red with the filter dropping every due job (no row at all).
a14bd84d74