The 16-year-old Union star isn’t the first wunderkind to be feted by the American soccersphere The trouble with trying not to hype up Cavan Sullivan is that he keeps on doing cool stuff. Like this past weekend, when he scampers along on a Philadelphia Union breakaway, loses his man by stopping so suddenly it’s as if he’s pulled a ripcord, back-heels the cutback behind his standing leg, watches it slowly kiss off the far post and in, and celebrated the goal by bowing and doffing an imaginary top…