Again and again, the music of Johann Sebastian Bach served as a place of refuge for Dmitri Shostakovich— especially during those times when the threat of the repressive Soviet cultural bureaucracy was at its most immediate. With his 24 Preludes and Fugues, he left a towering document of this artistic relationship: written in the early 1950s, these pieces reflect the composer’s deepest personal experiences underneath their seemingly abstract surface. Yulianna Avdeeva, most recently heard at the Pierre Boulez Saal with works by Liszt and Beethoven, performs the monumental two­hour cycle in a single evening.