Margot Robbie and Cillian Murphy — she the star of “Barbie,” he of “Oppenheimer” — have shared an experience, one unique in film history. On July 21, 2023, their two movies came out, and instead of cannibalizing one another during a time when box office receipts were sluggish, they actually boosted each other, creating the global phenomenon known as “Barbenheimer.” On paper, the two movies couldn’t be more different.