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 #6245, #6246, #6247 in one batch (quick gate green on merged tree, 5872 tests; 45s, gate 31s). 0 ejected. develop 1a842e8f6 = gated tree; stale develop_red record (686a2b39, vale) ignored as superseded.