The breakdown lane on the highway of basketball history is littered with the wreckage of college coaches unable to make a successful merge into NBA traffic. But clearly the Celtics feel confident that Brad Stevens is different. Actually, it’s the fact that he is exceedingly different in demeanor from most other college coaches that has the club believing he will prosper in the pros — and why, according to a league source, Stevens is receiving a six-year contract. While others are far more demonstrative, Stevens offered a calm exterior as he guided Butler University to two NCAA championship games and no fewer than 22 wins in each of his six years. And this is not to knock the sideline gymnasts who populate college ball. They are the constant in their programs, and the attention they draw is good for getting their team on the map and helping them recruit. While Stevens may be intense on the inside, his disdain for histrionics could help him wear better than the average college coach making the transition to an 82-game NBA regular season. Danny Ainge has always talked of his admiration for coaches such as Rick Pitino and John Calipari and the way they run their teams, but while the Celtics president of basketball operations chose someone who has also received high marks for basketball intellect, he is looking for a quieter solution to the team’s reconstruction.