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 #6575 (465556e44, fixes develop's red host_platform guard) and #6576 (ad0f8f1ee), both quick-gate green; none ejected. Develop green in the gate's jaahd container; queue empty. Memory: none applied.