From Ragtime, at City Center. Photo: Joan Marcus Over the past few years, as New York City Center’s program of musical revivals has inched toward this millennium, it has embarked on a grand journey into what I’ve come to realize was the thriving 1990s mini-genre of the Americana pageant musical: Parade , Titanic , and now, with this year’s fall gala, Ragtime . They braid fiction and historical events, move with a stately grandeur, and lean hard on AP U.S. History didacticism. With its enfolding