Killian Lassablière’s short documentary “Kukeri,” about a Bulgarian tradition intended to ward off evil spirits, is full of breathtaking, otherworldly images and sounds. Throughout the film, there’s a determined focus on the concept of legacy. “If you do not believe in something,” one of the interviewees says, in voice-over, “it cannot exist.” Traditions survive by being transmitted from generation to generation, like genes. That’s true of Kukeri, a custom that extends back centuries, so far int