On Oct. 21, 1958, Buddy Holly stepped beyond the guitar-driven sound that had made him a rock ’n’ roll pioneer and recorded four songs with orchestral backing in New York. The session offered a glimpse of a broader musical future that his death would soon cut short. Sixty-eight years later, it remains a defining moment in the story of an artist whose influence reached the Beatles, the Rolling Stones and generations of songwriters.Working with Di…