He was born 100 years ago on Aug. 25, and his centenary is being celebrated as his achievement — and the smilingly confident place and time he symbolized — seems ever more unrepeatable. Who today could write both “West Side Story” and three thorny, searching symphonies? Who could bring together Brahms and the Beatles on national television, and have millions watch? To what maestro’s left-wing political dalliances would New York magazine devote a cover story in 2018?