Follow-up to the salvaged #111044 commits:
- self_removal_delivery_allowed() now also requires that no record currently
holds the job id. The marker alone said "this run removed its record"; it did
not say the id is still empty. A replacement record (another owner reclaiming
the id) must be treated as a stolen claim, not a self-removal.
- Drop the allow_self_removed kwarg on fire_claim_fence: the fence already has
the job_id and the ContextVar marker, so it can decide on its own; the caller
no longer threads a flag it computed from the same predicate.
- _FireOwnership.lost(): keep the explicit lost event and the no-owner short
circuit ahead of the self-removal check so an interrupted run is still
reported as lost even after it removed its record.
- _finish_completed_run: skip mark_job_run entirely for a self-removed job
(nothing to mark) instead of calling it and then excusing the False.
- Tests trimmed to two invariants, both A/B'd against origin/main: the
self-removing run delivers after a post-removal heartbeat tick (RED on main),
and a self-removal followed by a replacement record is still discarded
(GREEN on main, guards the new predicate).
- Docs: user-guide cron.md notes that a job may remove itself and still report.
acb2c45e35