Rework RunDue/startRunLocked/executeRun so the ExecutionMode and
OverlapPolicy config knobs take effect:
- startRunLocked advances NextDue to the next occurrence instead of
zeroing it, keeping the schedule marching during an in-flight run.
- RunDue scans all due jobs: parallel starts every due, non-running job;
sequential defers a due job while any other job runs. When a job comes
due again mid-run, skip drops it and queue marks it Pending; either way
NextDue is advanced past the fired occurrence.
- executeRun re-runs a Pending job once the current run finishes.
- RunNow gains a sequential-mode guard refusing a manual run while
another job is running.
- Add anyRunningLocked and advanceNextDueLocked helpers.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
