LOOMIS — It’s a story that plays out often in California once the fall rainy season starts and mushrooms sprout: someone unintentionally picks and eats a poisonous variety, leading to hospitalization or even death. But Friday’s mass poisoning at an assisted-living facility near Sacramento, Calif., was shocking in its scope — two dead, four others sickened, including the caregiver who had prepared soup for residents using toxic wild mushrooms. Amateurs take a big risk when they harvest wild mushrooms, especially when they serve the fungi to others, said Casey Jonquil, owner of Alpine Foragers in Portland, Ore., who certifies and …