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 #6193 (9fca78c), #6197 (a61fccf) and #6198 (d18e60a); 0 ejected. Each lane batch was quick-gated green (5872 tests); develop at a61fccf passed the quick gate. Queue empty, no health breaches.