SOUTH BEND, Ind. — For many years, the conventional wisdom goes, anyone who has filed through the turnstiles of a professional baseball stadium and sat down to take in a game on a warm summer night has also risked being hit by a foul ball or the airborne splinters of a broken bat.But teams across the country now face a delicate balancing act — how to make the national pastime safer for fans without also ruining some of the fun — and that question arose again last week, when a line drive hit a woman in the head at a South …