I t’s here and then it’s gone . When the run of a play or musical ends, that’s it. If you’re a movie or TV aficionado, you can always make your way back through the canon to catch up. Theatergoers make do with fragments: cast albums, the tiny percentage of shows that were filmed for television, a fuzzy YouTube clip if they’re lucky, and whatever the New York Public Library for the Performing Arts caught on tape. Plus, of course, revivals. Much of this art form, instead, is retained in living