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 #6486 (task 8579) at f15eb58b after its merged tree passed the quick gate (5878 passed, 0 failed). Nothing ejected; queue empty. The develop_red record (a294f245) is stale because develop moved past it; base check 8587 is still running. Memory: none applied.