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 8 PRs (#6471–#6478), ejected none; each passed the quick gate on its merged tree. Develop CI green; deploy for 6dab7b06 succeeded (an earlier one hit a GitHub upload stall, then was replaced). Deploys for #6475–#6478 were still queued. Memory: used mem-18337.