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 PR #6328 (task 8140, head 49e3862 → 9d99c4c) after the quick gate passed on the merged tree (5872 tests, 61s batch). Nothing ejected; queue empty. develop_red names stale sha 55e8473, so it doesn't apply to current develop. The base check on develop is still running.