Who will be the first challenger for Jermell Charlo’s undisputed championship? 154 has rising stars who still present some danger. Jermell Charlo became undisputed champion at 154 lbs last night, beating Brian Castano in a fantastic rematch via 10th round stoppage to unify all four major titles in the junior middleweight division, the first men’s fighter to go undisputed in that division in the four-belt era. Charlo (35-1-1, 19 KO) has been at 154 since officially settling at the weight in 2011, after turning pro in 2007 as a welterweight. He still makes weight handily — he was cut and…