Three years ago, America’s largest pension fund made an unusual investment. It bought so-called tail-risk protection, a kind of insurance against financial catastrophe. In a market meltdown like the one sparked by the coronavirus, the strategy promised a massive payout -- more than $1 billion. If only the California Public Employees Retirement System had stuck with the plan. Instead, CalPERS, as the fund is known, removed one of its two hedges against a bear market just weeks before the viral outbreak sent stocks reeling, according to people familiar with its decision. The timing couldn’t have been worse.