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 #6516 (Task 8644) as 8de1b583: quick gate green, 5886 tests passed. Nothing ejected; queue empty, no health breaches. develop_red at 0aeacde is stale because #6515 later fixed that e2e. Batch took 51s (gate 35s). Memory: none applied.