TOKYO—Cars already know how to park themselves, warn drowsy drivers, steer back into the right lanes, and propose map routes to destinations. The cars Mazda has in the works for next year in Japan know when drivers have a stroke or heart attack. By 2025, the cars will even know when drivers are about to have a sudden health problem and warn them, according to the Japanese automaker. What’s involved are data from cameras inside the car, without resorting to laser sensors or other more obtrusive technology. And it’s going to be offered in affordable models, not just luxury vehicles. The technology holds promise for one of the most advanced aging societies in the world. Mazda told reporters recently it has been working with medical experts, including Tsukuba University Hospital, researching the collected image data to figure out what a healthy driver looks like, as opposed to an incapacitated driver, …