Blending documentary elements and some dramatic material (you don’t realize which is which until the movie springs its best surprise), “Radiant City” is an acerbic position paper on the cultural damage done by postwar architectural fads. The film unfavorably contrasts early-20th-century suburbs, which were built around shared public spaces and more conducive to pedestrian life, with their postwar successors, which lured inhabitants by promising huge amounts of space and no obligation to care about what happened beyond your property line.