In the dystopic locomotion of 2013’s Snowpiercer, Bong Joon Ho charted social hierarchy along the X axis. His new film, the Palme d’Or-netting Parasite, swaps horizontal for vertical, delivering a nasty update of the upstairs/downstairs formula for the South Korean service economy. It starts, quite literally, underground. A teenage brother and sister sprinting through the Kim family’s basement flat in a panic, trailing phones along the ceiling to snag a stray thread of WiFi. Their ground-level window offers a view of shoes and trouser bottoms, the alley’s pissed and pissing. The smog of city fumigators seeps in, fogging the domicile like…