The juxtaposition was dramatic, but not reflective of the big picture. About 20 hours separated the images of Tim Duncan in San Antonio and Dirk Nowitzki in Dallas. The contrast could not have been more striking. Duncan is a master at the art of poker face. But as he sat on the bench in the second half of the Spurs’ victory over Golden State on March 19, it did seem that a little bewilderment, or at least surprise, could be detected. That