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 #6580 (Task 8747) as 6cbcaeda7 after the quick gate passed on the merged tree (5965 tests). Nothing ejected; queue empty. The develop_red record names an older sha (stale); the new tip passes the quick gate. Time 50s (gate 35s). Memory: none applied.