BOSTON - With 3:19 remaining in the third quarter, with the Celtics up by 13, the crowd broke into an “M-VP” chant, acknowledging the player at the free throw line. But it wasn’t Jayson Tatum about to finish off a three-point play. It was Jaylen Brown, the star who has risen in the postseason, the player who is covering up for the curious amounts of time Tatum has been dormant in the series. …