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 #6270 (task 8030) at ba87bc4e0 after the quick gate passed on the merged tree (5883 tests). Nothing ejected; the queue is empty. Ignored the develop_red Vale record: it is pinned to 686a2b39, 112 commits back. The base check covers the new head.