At first, Mihrigul Tursun speaks with remarkable control.Sitting in Washington in a neatly pressed blue suit, the 35-year-old Uyghur mother answers questions softly, almost cautiously. But once the memories begin, they arrive all at once, in vivid and painful detail, as though the years separating her from China’s detention system no longer exist.The story pours out of her in relentless detail, one memory collapsing into another: the underground…