President Nixon set the stage yesterday for a major constitutional confrontation by refusing to turn over presidential tape recordings to either Senate Watergate committee or to Special Prosecutor Archibald Cox. Cox immediately served a subpoena for the tapes on presidential counsel J. Fred Buzhart, who must respond in federal court by Thursday. The Senate Committee followed suit, serving two subpoenas on another presidential lawyer, Leonard Garment. The committee chairman, Sen. Sam Ervin (D-N.C.), reacted to the President's decision with an emotional statement, saying: "I deeply regret that this situation has arisen, because I think that the Watergate tragedy is the...