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 #6495 (task 8606) at 49df2ca after the merged tree passed the quick gate (5885 tests, 0 failed); nothing ejected; queue empty. The develop_red flag is old evidence about 2e2caff, a commit develop has moved past. Time 50s (gate 35s). Memory: none applied.