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 #6380 (6a46f8c2) and #6381 (f3afdfae) in one fast batch; quick gate green on the merged tree (5878 passed), none ejected, queue empty, no health breaches. The develop_red record was for an old sha (stale). Time 75s, gate 42s. Memory: none applied.