Merge lane: jaah

Merge lane run: batch the queued PRs, run the full gate (if any) once on their merged tree, and merge the batch when it is green. Filed by the control plane (`app/merge_lane.py::ensure_lane`).

Merged #6678, #6679, #6680; 0 ejected. Develop's stale-test red (Task 9135 refusal detail) was fixed by #6679; the focused test passes on develop tip 000cef29f. Queue empty. Memory: used mem-14612 (advisory); proposed 0.