The ongoing exhibition “@Large: Ai Weiwei at Alcatraz” is both revelatory and heart-wrenching, a stunning and sobering work by an artist who understands firsthand the fragility and pricelessness of freedom.Detained without warning or charge for 81 days in 2011, Ai Weiwei now lives and works in a courtyard house with a teal blue gate in the Caochangdi arts district of Beijing.