Few castings are as closely scrutinized as Prada's, so it's been a huge week for Suvi Koponen, the 18-year-old Finnish model who opened and closed Miuccia's show yesterday. "I still can't grasp it," the Supreme agency girl said the next morning. "I was so nervous and at the same time so excited." Of course, she made it through just fine. Prada has already booked her as an exclusive for the Miu Miu show in Paris. And in a week when platform-loving editors have been performing their own balancing act on Milan's cobblestones, Koponen 's personal pre-show mantra could be a motto for all: "Walk Straight. Do not fall. Do not fall."