MartÃn RamÃrez, a Mexican immigrant who lived in a California mental hospital for more than 30 years, was also one of the more visionary “outsider” artists of the last century. So when “a cache of some 140 of [Ramirez’s] drawings, all from the last three years of RamÃrez’s life, many of them dated and most in great shape,” was discovered in a California garage recently, curators at the American Folk Art Museum were on the next plane.