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 #6518 (782a505) and #6519 (38dec5e); none ejected. The develop_red record pinned to 8de1b58 was stale: a focused full gate on develop passed (55167 tests, 0 failed). Queue empty, develop green. Memory: none applied.