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 #6695 (Task 9185) → 0c474ec4b9 after quick gate green (6014 passed). First try requeued on a transient forge read of the lane's own pending status; retry merged. Nothing ejected; queue empty, develop not red. Time 50s: gate 31s. Memory: none applied.