The southeast facade of the U.S. Federal Building in San Francisco is covered with a perforated stainless-steel scrim that seems at once to be a diaphanous veil and a sharp-edged protective shell. And the dual nature of this 18-story office tower seems just right for its rapidly changing but still gritty environs, where pawn shops sit cheek by jowl with luxury condos. Completed in March, the 240-foot-tall tower dominates the mostly low-rise South of Market skyline and is reportedly snarling traffic on nearby Interstate 80 as drivers slow down to take a look. But its height and gutsy exterior are not the only reasons the Federal Building is getting attention. It also has a set of ambitious environmental goals.