Every few years, a genre-bender comes along, breaking some of those conventions but maintaining the essential ethos of that genre. “Parasite,” Bong Joon-ho’s haunting depiction of class struggle in South Korea, doesn’t fit into either category — the film neither conforms to nor breaks free of genre convention. Instead, Joon-ho’s latest thriller is indubitably a masterful genre-melding exercise, gaining the audience’s trust and comfort only to shatter the carefully constructed world later on.