DALLAS — For a building that’s been “rowdy, proud and loud” for two decades, for the video that now streams before fourth quarters — of Kyrie Irving yelling, “Don’t be boring!” — the 20,555 watching Dallas stave off Oklahoma City fell into a chilling silence as P.J. Washington stepped to the foul line with 2.5 seconds remaining Saturday night. A one-point deficit separated the Mavericks from a trip to the Western Conference finals, and Washington, stoically, drained his first of three shots. And then, well, American Airlines Center famously positions blistering hot microphones above the rims of each basket, which boomed each ricochet of Washington’s second attempt, his game-winning attempt, amplifying the live theater playoff basketball can truly become as a stadium holds its collective breath.