An occasion supposed to herald an age in which the nuclear threat becomes a thing of the past was marked by considerable grumbling between two cold-war foes. Even as President Clinton, on a quick stop in Budapest, Hungary, welcomed the kick off of the Strategic Arms Reduction Treaty with a schedule for scuttling 9,000 U.S. and former Soviet nuclear warheads by the turn of the century, Boris Yeltsin complained about NATO's vote last week to consider membership for former Warsaw Pact nations. "Why sow the seeds of distrust?" Yeltsin rhetorically asked. "After all, we are no longer enemies. We are all partners." He warned that NATO's action could force progress against the Cold War to "sink into oblivion." A Clinton Administration official dismissed Yeltsin's criticism as "alarmist."