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 #6451 (Task 8399) at 7e85629, quick gate green on merged tree (5878 tests, 55s; gate 39s). None ejected; queue empty. Develop-red flag is stale (c975b8a; muted-text test fixed by #6450); base check pending. Memory: none applied.