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 PR #6300 (task 8086) at 2e8388f1 after the quick gate passed on the merged tree (5872 tests). Nothing ejected; queue empty. Not acted on: develop_red names 686a2b39, an older commit. Batch took 66s, 45s of it the gate.