American freestyler Katie Ledecky has never gone home with anything other than a gold or silver medal in the Olympics or world championships dating from the 2012 London Games. But it might happen Sunday in the women’s 400-meter freestyle on the opening night in the pool of the World Aquatics Championships. This is perhaps the most anticipated race of the entire eight-day program in Japan. Ledecky is up against two younger stars. There is 16-year-old Summer McIntosh. She is a Canadian who holds the world record at 400. And 22-year-old Ariarne Titmus. She is an Australian who is also a former world-record holder.