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 #6489, #6490, #6491, each quick-gated green on its merged tree (5889 tests); nothing ejected; queue empty. develop_red e2e evidence is on stale a294f24; base check 8587 open, no breaches. Memory: none applied.