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 #6408, #6410, #6412, #6413, #6414, #6415 (each tree quick-gated green, 5889 tests). Ejected #6409: conflicts with develop (dirty), back to its runner. develop 511155f53 green on quick gate; full base check running. Memory: none applied.