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 #6581 (quick-gated merged tree, 5965 passed); 0 ejected; queue empty. It fixed develop's red test_node_orders (from #6580); now 63/63 pass on develop 111e3896. Batch time 36s (gate 21s). Memory: none applied.