With a handful of mega-cap tech and AI stocks at the top of the S&P 500 Index dominating the U.S. market in a way without historical precedent, portfolio concentration risk has taken on a new form for investors long told to follow some version of Warren Buffett's stock advice to "never bet against America."But with the nine tech stocks that are above Buffett's Berkshire Hathaway by weight in the index representing nearly 40% of the market, it's an imbalance that has investors looking for new ways to hedge. ...