Those who have been in New York long enough know the deal. Reggie knew it. Jeter lives it. So does Eli and Messier and anyone who played from the 1969-70 and 1972-73 Knicks. It’s what you do in the playoffs that shapes your legacy in New York. That’s how an athlete becomes legendary in this town, even if his team plays in Brooklyn. Deron Williams has a chance to achieve what Jason Kidd never could while playing for the Nets and leading his team to back-to-back NBA Finals in 2002 and 2003. Because the Nets played in New Jersey, Kidd never was revered in New York the way he would have been if he were a Knick. Maybe it would be different if Kidd and the Nets actually won a title, but I think not. He wouldn’t be remembered the way we remember Willis, Clyde and Bradley for the championships they won. The New Jersey Nets never were going to get any New York love. It could be different now that the franchise is in Brooklyn. It could be different for Deron Williams. Now he has a chance to build his Brooklyn/New York legacy, one playoff win at a time. “It’s exciting to be back in the playoffs,” Williams said yesterday at the Nets practice facility in East Rutherford, N.J. “The whole team is excited about the opportunity to be in our first year in Brooklyn and being able to give the fans a playoff experience.”