HARDAN, Iraq — Peering through binoculars, the young man watched as Islamic State extremists gunned down the handcuffed men and then buried them with a waiting bulldozer. For six days he watched as IS filled one grave after another with his friends and neighbors. The five graves arranged at the foot of Sinjar mountain hold the bodies of dozens of Yazidis killed in the Islamic State group’s bloody onslaught in August 2014.