SCIENTISTS DETONATED a nuclear weapon Friday after a 21/2-hour technical delay, delivering a blast that was felt in Las Vegas high rises, 105 miles from ground zero. Friday's test was the seventh of the year and the third in three weeks at the sprawling Nevada Test Site . . . DEMONSTRATORS at the Capitol in Washington, D.C., marked Friday's anniversary of Columbus' discovery of the New World by beginning a fast to protest what they said were violations of Indian rights. A group called Fast for Big Mountain said six people would engage in a water-only fast on the Capitol steps until Nov. 22, Thanksgiving Day.