ATHENS — If you are Jeff Bezos, you probably don’t need to read this. That’s because if you’re a billionaire preparing to gallivant around Europe this summer, chances are you’ll be doing it at least partially aboard a yacht. For the rest of us, refreshing sea breezes are not a given as the Continent girds for what is shaping up to be the hottest summer on record. Tourists, Americans among them, are facing a silent killer in the form of heat stress.