In one of the most moving scenes in "Billy Elliot," a young boy meets his older self in a dreamlike duet — his hesitant, searching steps echoed and lifted by graceful, assured movement. For a brief moment, the two dance as one before drifting apart, tracing a quiet arc of longing, distance and the self he is still becoming. The scene — among the production’s most striking moments — was performed on Wednesday at a press call. Lim Sun-u, a principal dancer with Universal Ballet, played the adult B