They stood on opposite sidelines Friday night at TD Garden. Mike Brown coached the Cavaliers, Brad Stevens the Celtics. But they had an unusual – and, to be fair, rather uncomfortable – connection: Brown’s son, Elijah. In April, Elijah Brown, a shooting guard out of Mater Dei High School in Santa Ana, Calif., signed a national letter-of-intent to play at Butler for Stevens, then the head coach of the Bulldogs. Before the ink was dry, though, Elijah’s father called Stevens just to feel out the coach, who had been rumored to be leaving.