Jessica Chastain surprised audiences at the Cannes Film Festival on Thursday, when she appeared in an unannounced cameo in the 1980s coming-of-age drama “Armageddon Time” as Maryanne Trump, Donald Trump’s sister.Chastain’s role is small but effective. As Maryanne Trump, she appears as a guest lecturer at an austere private school where the film’s young protagonist, Paul (Banks Repeta), matriculates mid-film. She lectures the privileged boys and girls about the value of ambition in a Phyllis Schlafly-esque beehive.