Lady Caroline Lamb called her lover, the poet George Gordon — the Sixth Baron Byron — “mad, bad, and dangerous to know.” Now the aristocrat of centuries past is the center of an exhibit, “Byron: A Life in Motion,” at the New York Public Library. It draws on a store of treasures relating to the Romantics. Byron was a celebrity avant la lettre, a sensation and a scandal. A poet and a peer, he is in the firmament of English literature.