KILLINGTON, Vt. — The problem with planning for milestones in ski racing is that calamity can feel more likely than joy. It looms at every turn. So it was that Mikaela Shiffrin, the heavy favorite to claim the 100th World Cup win of her career here, instead spent her Sunday on a couch down the hill from the racecourse, talking about the excruciating hole in her side and the prospects for her immediate future.