Plenty of dancers who worked with the great choreographer George Balanchine have gone on to lead their own companies. None has been a more devoted keeper of the flame than Suzanne Farrell, Balanchine's muse and most brilliant ballerina for some two decades at the New York City Ballet. To launch its new season at the Kennedy Center in Washington, D.C., Farrell's eponymous troupe will bring back one of its specialties, "The Balanchine Couple," an evening of nine pas de deux from the Balanchine repertory. And to serve as onstage narrator, tracing the evolution of the master's style, who better than the artist who danced many of the roles so memorably, Farrell herself?