By Winni Zhou and Mari Saito ZHANGJIAKOU, China (Reuters) -American Chloe Kim cemented her position as one of the greats of women’s snowboarding with a commanding performance on Thursday to win halfpipe gold at the Winter Games and successfully defend her 2018 Olympic title. The 21-year-old set herself apart from rivals in the very first run, earning a top score of 94 by breezily landing 1080s. Spain’s Queralt Castellet could never catch up with Kim and had to settle for silver, while Sena Tomita of Japan won the bronze medal. Kim, who at 17 became the youngest woman ever to win Olympic gold i…