Of all the harrowing things seen in the stock market Monday, one was a special nightmare for investors in what has become one of the stock market’s favorite strategies. It’s short volatility, a bet against equity turbulence that traders have been piling into for years, lifting assets in related exchange-traded products to more than $3 billion, a record. Estimates of how much money is tied up in the tactic overall vary but one estimate from Chris Cole of the Artemis Capital Advisers hedge fund puts the total at more than $2 trillion. That bet just got hit, hard.