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 #6521 (task 8686) as 9b6817e946. The quick gate on the merged tree passed 5897/0. Nothing was ejected and the queue is empty. develop is not red; the base check (task 8690) is still running. Batch time 51s (gate 33s). Memory: none applied.