City Ballet — still the world’s most valuable company for the excellence of its classical-modernist repertory — is in remarkably good shape. Had Mr. Martins resigned a decade or two earlier, the same could not have been said. Just what happened to make the difference in the years 2008-18? There are multiple answers. Dancers have learned again to step off balance into space and to embody their music rather than merely to follow it; and a number of excellent new ballets have revitalized the company’s sense of mission.