To put things into perspective: 10 stocks have contributed 90% of S&P 500's 13.0% gain this year, the most concentrated mkt ever, Bernstein says. The stocks are AAPL (up 39%), MSFT (+36%), NVDA (+164%), GOOG (+38%), AMZN (+48%), META (+120%), TSLA (+91%), AVGO (+45%), AMD (+87%)…