Those interested in the future of downtown Dallas may wanna carve out some time on Friday -- like, all day Friday. Because from 8:30 a.m. to 5:30 p.m., City Hall will play host to a daylong "design charrette" during which city officials; architect Brent Brown, founder of bcWORKSHOP; and folks from San Francisco-based Urban Re:Vision will hash out how to turn a single block of downtown into "the first fully sustainable, urban square block in the United States." Seems a mighty tall order, and it i