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 #6141, #6153, #6151, #6154, #6156; develop 6c0b98c quick-gated green. Ejected #6155 (Python lint ruff failure) and #6150 (conflict). Queue empty. develop_red at c2ad861 is stale: vale + ast-grep-pin pass on 096a62a.