If you buy Apple's new $1,999 folding iPhone this Friday, a bunch of your apps are going to open as a tiny phone-sized rectangle floating in a sea of black. Apple knows. It set it up that way on purpose, and it already picked the month it ends. Six weeks ago, Apple's new CEO John Ternus got up at the launch event and poked fun at Android foldables for doing exactly this. The way it works on the Duo is pretty simple. Whether an app fills that big inside screen depends on which version of…