While it’s tempting to keep riding the wave, investors would be wise to diversify their portfolios, says Jonathan Levin for Bloomberg Opinion.  MIAMI: The latest earnings season is drawing to a close, and it’s revealed how the artificial intelligence boom is engulfing more and more of the S&P 500 Index. The profits are pouring in, yet the US is increasingly a single-narrative market - one where stocks will swoon together when the music stops playing. The overwhelming majority of sectors is