Apple is finally opening the iPhone to third-party app stores in the European Union, kicking off a potentially vibrant, unwieldy, and eclectic new era for its app ecosystem. At least, it might, depending on how developers respond to a hurdle that is at once tiny and immense: a €0.50 fee.Apple is introducing a new fee structure for apps that want to operate on these third-party stores. On the surface, it looks great: apps pay no cut of sales to Apple if they’re distributed via a third-party store...