WASHINGTON — Every disaster has its the moment of clarity. Physicist Richard Feynman dunks an O-ring into ice water and everyone understands instantly why the shuttle Challenger exploded. Last week, the Obamacare O-ring froze for all the world to see: Hundreds of thousands of cancelation letters went out to people who had been assured a dozen times by the president that “If you like your health care plan, you’ll be able to keep your health care plan.