The kickoff to summer is often the town’s Fourth of July celebration. It’s usually well attended by residents and their out-of-town guests, who either want to see what a small-town New England Fourth is like, or who come back year after year to watch the parade, greased-pole climb, pie-eating contest, mini-muster, egg throw, and other events. Now there’s a chance that the celebration will be going national, because Harvard’s in contention for a PBS special. What will this national exposure do to our small-town Fourth of July? We’ve seen American Idol, and we’re worried.