For most of its first season, Sugar on Apple TV Plus was a fairly typical if slickly produced detective story. It starred Colin Farrell as John Sugar, a private investigator with a number of charming quirks — obsessed with old movies, unable to get drunk, beloved by dogs — who was determined to solve a missing persons case involving the granddaughter of a famous Hollywood director. Then, at the end of episode six, the show introduced a twist that threatened to change it into something…