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 #6232 (task 7950) as 878fade6 after a green quick gate on the merged tree (5872 passed). Nothing ejected; the queue is empty. The earlier develop red on docs-prose-lint-vale does not reproduce: Vale exits 0 on 878fade6.