Phase 4B. Three NON-abstract hooks on the CronScheduler ABC, all with
built-in-safe defaults so the built-in inherits them without overriding and
test_abc_growth_stays_additive stays green (required surface still {name,
start}):
- on_jobs_changed(): post-mutation reconcile hook. Built-in no-op.
- fire_due(job_id): claim the job via the store CAS (claim_job_for_fire,
Phase 4C) then run it through the shared run_one_job (Phase 4A). Returns
False if the claim is lost or the job vanished (repeat-N exhausted between
arm and fire). The inbound webhook (Phase 4E) routes here.
- reconcile(): converge the external registry toward jobs.json. Built-in no-op.
fire_due imports claim_job_for_fire/get_job/run_one_job INSIDE the method, so
this commits cleanly before Phase 4C lands claim_job_for_fire (import-time is
unaffected; tests monkeypatch it with raising=False).
Tests: required-surface-unchanged guard, built-in inherits no-op defaults, and
fire_due's three paths (claim+run, lost-claim→no-run, missing-job→no-run).
tests/cron/ green (20 in test_scheduler_provider.py).
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