OK, I could swear I predicted this months ago, but maybe I just have the vapors: Paul Giamatti is joining the Season 4 cast of “Downton Abbey,” as Lady Cora’s playboy American brother, Harold Levinson (son of Martha, played by Shirley MacLaine). Those two as mother and son sounds just about right, doesn’t it? Harold has been mentioned but never seen in previous seasons; he holds the other half, presumably, of the Levinson fortune. So he’ll be showing up at Downton, presumably, all American and sneery and Paul Giamatti-ish (which is a fine thing, to be sure), scandalizing the Dowager Countess, and this all sounds like good fun. Who needs Matthew?