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 #6418 (Task 8336) at 8bb6a083 after a green quick gate on the merged tree (5889 passed, 52s). Nothing ejected, queue empty. develop shows no breaches; the develop_red entry is on stale sha f1524694, which #6417 fixed. Memory: none applied.