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 #6481 (Task 8183) as 43ec7be0 after a green quick gate (5889 tests passed). Nothing ejected; queue empty. The develop_red record points at stale 171d4161, which #6480 fixed; the quick gate is green on the new develop tip. Batch took 50s (gate 32s).