Architect Rob Wellington Quigley has done more than fret and attend symposiums about afordable housing. He has designed and built four low-rent, single-room-occu- pancy hotels. The latest is his best: 197 airy rooms that cost only about $20,000 apiece to build, landscaped courtyards and a reading room. The three- sided exterior is magnificent by subsidized-housing standards, particularly a kooky De- constructivist façade in bright purple and red.