Nothing about this traditional design says "gray water treatment happens here." Renderings: Southern Heritage Homes Lined with rundown, century-old houses and situated within a couple miles of downtown Roanoke, the neighborhood of Gainsboro, Va., seems an unlikely place to hatch a groundbreaking architectural experiment. But in early November, construction will begin there on the first cradle-to-cradle house, with those behind the project hoping to show that green can be affordable. In many ways, Gainsboro is the perfect site for such an undertaking. After suffering years of deterioration and failed urban-renewal efforts, the city's oldest neighborhood had been targeted by municipal …