Pay $1,999 for Apple's new folding iPhone, open Apple's own Apple Store app, and you get a little phone-sized window stuck in the middle of a big black screen. Apple built those black bars on purpose. Once you know why, you'll be able to tell which of your apps are going to look like this for a while. Here's how it works. Every iPhone app gets built with a certain version of Apple's developer tools, and iOS looks at that version before it puts the app on screen. If your app was built before…