The citizens are staging an intervention. In the world’s growing metropolises, local problems increasingly are being addressed through street-level collaboration. Think community farms, bike lanes, street markets, product- and service-sharing, and even temporary purpose-built structures. The U.S. Pavilion at the 13th International Architecture Exhibition of the Venice Biennale, which kicks off Aug. 29 and ends Nov. 25, will showcase more than 100 of these projects (whittled down from 450 submissions), mapping out for visitors a new turn in the life of the American city and offering a glimpse of what’s ahead.