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 #6342 (task 8198) as 9287e495 after its merged tree passed the quick gate (5877 tests). Nothing ejected. Queue empty, develop green; the develop_red mypy record on 55e8473 is stale, and mypy is clean on the new head. Batch: 61s, gate 43s.