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 #6416 (task 8283) and #6411 (task 8271) into develop; quick gate green on merged tree (5878 passed). Nothing ejected; queue empty. Stale develop_red (older sha f1524694) superseded. Batch time 50s (gate 31s). Memory: none applied.