Now that you’ve created the next great app and your investment in your startup is worth a billion dollars, at least on paper, it’s time for a really difficult challenge: how to cash out without paying capital gains taxes anywhere in the world? With no U.S. or Puerto Rican tax on much investment income of new residents, U.S. citizens now can pay zero tax on capital gains. For several hundred affluent Americans over the last couple of years (including Toby Neugebauer, a key financier to Ted Cruz) the answer has been to establish a principal residence in Puerto Rico, and...