The U.S. Figure Skating Championships are coming to Boston January 5 to 12, with main events at the TD Garden. The 2013 U.S. pairs champions will be on home ice: Simon Shnapir, 26, grew up in Sudbury after moving from Russia as a tot, and Marissa Castelli, 23, lives in Allston. The duo train at the Skating Club of Boston, and, if they win, are headed to Sochi. They’re the favorites—even if they haven’t always been each other’s favorite people.