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 #6682 (task 8669) → 2187c266 after quick gate green on merged tree (6014 passed). 0 ejected; queue empty. develop_red evidence names stale ancestor 3e6be770 (fixed by #6679); base check on 000cef29 still open. Memory: none applied.