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 #6424 (Task 8354) → cd76fe8 after quick gate green on merged tree (5889 passed); 0 ejected; queue empty. develop_red record is for stale bbedc19, owned by base check 8352. Batch time 48s (gate 31s). Memory: none applied.