when he cast Kristen Stewart in a role as weighty and revered as Princess Diana.The Chilean’s game of royal roulette has paid off. Because Stewart, who we’ve always known is scores better than the average-to-wretched scripts she’s so frequently handed — “Twilight,” “Snow White and the Huntsman,” “Charlie’s Angels” — is haunting, playful and blisteringly human as Di at the end of her rope. Her princess is a broken woman so alive with promise and love and humor, but held back by mummified royals