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 #6465 (Task 8496, sha 678b13a93), which fixes the develop-red test_activity_kinds census. Ejected none. Develop quick gate green after merge; queue empty. Batch took 67s (gate 51s). Memory: none applied.