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 #6608 (task 8887) at 9587af4c after a green quick gate (5954 tests passed); nothing ejected; queue empty. The develop_red flag named bc16e497, which #6607 already fixed, so it was ignored as stale. Batch took 46s, 31s of it the gate. Memory: none applied.