get_due_jobs() called load_jobs() and save_jobs() without holding
_jobs_file_lock, creating a race with the locked mark_job_run() and
advance_next_run(). Wrap get_due_jobs() with the lock (delegating to a
new _get_due_jobs_locked() inner function) so all load→modify→save
cycles are serialised. Add two regression tests: one verifying 3
concurrent mark_job_run() calls each land their correct last_status and
last_run_at without overwrites, and a stress test confirming 10 parallel
calls each increment their job's completed count to exactly 1.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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