PARIS — Karl’s khakis — they’re as American as apple pie. And so is the rest of the witty, exuberant collection Lagerfeld showed for Chanel Thursday. In it, he gave a nod to preppies, suburbia and even a mall rat or two. Of course, these are rich preppies and mall rats who can afford to shell out Chanel prices for a pair of hip-riding chinos like these — and have enough left over for one of Karl’s great new, loosened-up tweed jackets. For more on Paris, see pages 8 to 10.