With all the clout comic-book movies achieved over the last two decades, it’s easy to forget how low they ranked on the cultural food chain a half-century ago. Taking that view from outer space, the twin arrival of “Superman” and “The Fantastic Four: First Steps” bookends an era in which superheroes soared to unprecedented heights and, depending on how this pivotal month plays out, risk falling back toward Earth. Understanding the current state of the superhero movie actually requires