HISTORICALLY, Boston looms large, but geographically it's just a bean of a town: 51 square miles, with more than 20 distinct neighborhoods crammed cheek by jowl, with pricey Beacon Hill and gritty Chinatown facing off across Boston Common, high-brow art museums just a stroll from Fenway Park. That rich density makes even a short walk rewarding, and the beginning of the end of the Central Artery-Tu