"“Beau Is Afraid,” a mommy-obsessed magnum opus from elevated-horror auteur Ari Aster (“Hereditary,” “Midsommar”), opens with a therapy session, conducted virtually in real time, in which the title character — played by a balding, bleary-eyed Joaquin Phoenix — shares his anxieties with a sympathetic clinician (Stephen McKinley Henderson).
Beau is scheduled to leave the next day to visit his mother, a fraught proposition that leads to the shrink wondering aloud whether Beau has ever harbored matricidal fantasies; within a few…