Shakespeare’s “King Lear,” so grand in its scope, so dark in its vision, is not a play about a single man. Playing the maddened and maddening title character, Christopher Plummer gives a performance of such purity and truth that it lays instant and entire claim to our hearts and minds; there’s not much compassion left for anyone else, so powerful is our engagement in Lear’s physical and intellectual dissolution amid the harrowing storms of life.