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 #6532 (Task 8725) as 9ae075f4 after the quick gate passed on the merged tree (5899 tests, 33s). Nothing ejected; the queue is empty. The develop_red record pointed at stale sha 5f27a552, so it was ignored. Memory: none applied.