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 #6523 (task 8681) at 6abcb16d after the quick gate passed on the merged tree (5886 tests passed, 0 failed). Ejected nothing; queue is empty with no breaches. The develop base check (task 8690) hasn't finished. Batch time 50s (gate 32s). Memory: none applied.