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 #6615, #6616, #6617 into develop in 2 quick-gated batches (5974 tests green each); none ejected; queue empty. #6615 fixed the real e2e failure in the f49b90d red; the other 3 were flakes per Task 8952. Memory: none applied.