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`).

Merge lane: queue empty (lane.queue/lane.batch rows=[]), nothing merged or ejected; no gate run needed. develop at 385c73571 not flagged red.