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 #6597–#6602 (6 PRs), ejected none. Fixed red develop forward in #6601 (stale goldens/digests + leaked describe_instance stub). Full gate on 7851c60 green except runner-cli-lock-audit (npm registry unreachable from host). Deploy succeeded. Memory: proposed 1 gotcha (mem-23528).