Photo:  Antone Patterson An opera company’s highest calling is to commission new work—to contribute to the history of opera, to leave a legacy. Seattle Opera has done so, memorably, with “A Thousand Splendid Suns.” The piece, which opened February 25 in Seattle’s McCaw Hall and ends March 11, is Seattle Opera’s first mainstage world premiere since Daron Hagen’s “Amelia” from {…}