The wounds of abandonment and displacement are at the root of Zou Jing ’s achingly poetic “ A Girl Unknown ,” a sober and quietly devastating portrayal of an adolescent girl whose identity, maybe even humanity, has been toyed with by a fractured law. That would be China’s controversial one-child policy, recently interrogated in Nanfu Wang’s stunning documentary “One Child Nation.” While “A Girl Unknown” isn’t directly an examination of the severe initiative that was introduced in 1979 to