THE COLD WAR was supposed to have ended 15 years ago, but the death in London of Alexander V. Litvinenko presents Scotland Yard with more than your average murder mystery. The former Russian spy and fierce critic of the Kremlin was poisoned. But how and by whom? The tale began Nov. 1 at itsu, a busy London sushi restaurant near Piccadilly Circus that features a Madame Butterfly Zinger, Squirrels Dreams and something called Bang Bang Free Range Chicken. Poison is definitely not on the menu. Litvinenko, an outspoken critic of Russian President Vladimir V. Putin and a KGB successor agency,...