Pianist Pierre-Laurent Aimard returns to the late-night Mostly Mozart series with an intriguing program of preludes by D'Anglebert, Mozart, Bach, Chopin, Debussy and Scriabin, canons by Schumann, Brahms, Bach, Webern, Boulez and Ligeti, and a work by George Benjamin, whose "Written on Skin" is on the festival's main stage this week.