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 #6492 (task 8600) at 778fdd5 after a green quick gate on the merged tree (5878 tests, 71s: gate 56s, merge 6s). Nothing ejected; queue empty. develop_red points at old sha 5cd29f7, so it's stale; a base check runs on develop. Memory: none applied.