The “slap heard ‘round the world” has come to define this year’s Oscars, which aired March 27. Comedian Chris Rock was presenting the award for best documentary and preceded it with a monologue that included a joke about Jada Pinkett Smith’s hair loss, which is due to her alopecia. At first her husband, actor Will Smith, seemed to laugh at the jab. But then he abruptly strolled on stage and smacked Rock across the face.…