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 #6310, #6312, #6313, #6314 (each quick-gated green, 5872 tests); nothing ejected. Queue empty. develop 55e8473 green: the stale develop_red (Vale on 686a2b3) re-run passes.