“The Magnificent Ambersons,” Orson Welles’s follow-up film to “Citizen Kane,” about a wealthy Midwestern clan brought low, came with a bewitching backstory: the studio had ripped the movie from the director’s hands, slashed 43 minutes, tacked on a happy ending, and destroyed the excised footage in order to free up vault space, leaving decades’ worth of cinephiles to obsess over what might have been. In September, the startup Fable Studio, led by the businessman Edward Saatchi, announced that it