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 #6507 and #6512 (each quick-gated green on its merged tree, 5897 tests). #6508–6511 were already merged. Nothing ejected; develop not red, queue empty. Memory: none applied.