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 #6458 (132f272) and #6459 (3f59ab1) in one batch; quick gate green on merged tree (5878 passed). Nothing ejected; queue empty; develop not red. Time 62s (gate 34s). Memory: none applied.