IT’S AUGUST IN Washington, and the livin’ ain’t easy. Under the swelter of the noonday sun, the nation’s capital is moving in shimmering slow-mo. If you walk, you’ll wilt. The subway tunnels are steam baths. Most of the city’s taxicabs — those without air-conditioning — are hell on wheels. The smart folks have gotten out of town. Had it not been for Harry Zitelman, however, things might be even worse.