The Giants’ win over the 49ers in a magnificent throwback conference championship game at Candlestick Park yesterday turned on two fumbles by 49ers punt returner Kyle Williams, an obscure second-year player. That made Kyle Williams personally responsible for ten points in a game that ended 20-17. And the Giants, interviewed in the happy haze of the winning locker room, casually noted a provocative element of their game plan: They’d targeted Williams for extra violence because they knew he had suffered several concussions in the past, and they think it worked.