François Couperin's Leçons de Ténèbres, set to the Lamentations of Jeremiah and intended for use on Thursday of Holy Week (they may have been part of a larger set, now lost), would seem on cursory hearing to be light-years removed from his glittering keyboard works, so redolent of the hothouse atmosphere of the French court. Listen again, however, and you find connections: Couperin transfers his uncanny way of making an ornament hang in the air to these deeply serious, arioso settings of the Lamentations for one or two voices, plus continuo…