The barrel-roof townhouses of River Park are Campbell’s soup cans, halved the long way, balanced on top of metal cubes two blocks from Southwest Washington’s Waterfront Metro station. They are called “houses” — because people live in them, and really, what else could they be called? — but they are architectural punch lines, visual acid trips, the left-behind parts of the secret UFO that docked down by the waterfront half a century ago and then flew away before anyone caught it on camera.