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 #6466, #6467, #6469 (quick gate) and fix-forward #6468: #6466 had turned develop red (switch-matrix test missing unit.body). Full gate green on the fix; none ejected. #6468 deploy superseded by #6469's. Memory: proposed 1 gotcha (mem-22667).