CANNES, France — The audience made barely a sound for almost 2½ hours at Friday’s premiere of Ari Aster’s slow-then-deranged American political horror story “Eddington.” No laughing. No gasping. Just silence, watching the A24-anointed director’s dark comedy, set in small-town New Mexico at the start of the pandemic, in which a personal feud between the MAGA-coded sheriff (Joaquin Phoenix) and the liberal mayor (Pedro Pascal) turns into a culture war over mask mandates — and then actual war.