Jin Jiang’s documentary “Republic” elicits frustration, largely because it sets up a veritable obstacle course of contradictory emotions. As a cinematic venture, the picture is deeply disturbing even as the pathologies it underlines are rote. Adolescents, we learn, are ever thus even as the age of adolescence seems to be extending beyond its chronological sell-by date. Add to this fraught dynamic one slacker’s notion of revolution and you have a picture that is, alternately, an expose, a parable, and a snuff film.