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 #6452, #6453, #6454 (quick-gated green, 0 ejected); queue empty. Develop full gate red on 3 pre-existing Playwright smoke specs (base check 8404); fix task 8411 already active. Memory: none applied.