To Americans eager for signs of life in an ailing cinema culture, the simultaneous box office success of the “Barbie” movie and the biopic “Oppenheimer” has been cause for celebration, with filmgoers embracing the jarring juxtaposition of the two very different blockbusters. In Japan, however, this jubilant fusion, including “Barbenheimer” double features and online mash-ups of Barbie’s pink fanta