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 #6672 (task 9006), #6673 (9110) and #6674 (9129) into develop. Batch quick gate green with 6014 tests passed; nothing ejected. Develop tip 2867d9327 passes the quick gate and the queue is empty. Memory: none applied.