Xibe farmworkers in a cotton field near Qapqal County, China, in October.Credit...Adam Dean for The New York Times QAPQAL XIBE AUTONOMOUS COUNTY, China — Loyal to the core and prized for their horsemanship, several thousand Manchu soldiers heeded the emperor’s call and, with families and livestock in tow, embarked in 1764 on a trek that took them from northeastern China to the most distant fringes