Since 1954, sugar plums have danced in the heads of American youngsters enchanted by the magical tale of Clara and her nutcracker prince. The music and the dancers whirling across the stage in a lavish setting have made hearts beat a little faster at Christmas time for three generations. But no matter where it was performed in this country, the production was predictable. Everything happened on stage exactly as it had happened for years. Enter the brilliant Russian choreographer Alexei Ratmansky. When called upon last year to direct the American Ballet Theatre in “The Nutcracker,” he remembered how bored he had become with the story at the start of his career in Russia. Proclaiming that “the magic disappeared,” he set out to reclaim the lost magic. In the process, he made more sense of the second act, thereby creating a romantic version befitting today’s modern audience.