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 #6463 (8df4e5c7) and #6464 (afea52ee), each quick-gated green on its merged tree. Nothing ejected; queue empty. The stale develop_red tests passed on current develop (re-gated). Time: gate is the largest phase (36s/26s). Memory: none applied.