Smart horror–is that an oxymoron? Not in “The Cabin in the Woods,” a devilishly twisted film written by Joss Whedon, maker of beloved TV series “Buffy” and “Firefly.” Whedon starts with a generic plot premise that has been hackneyed to death: youth in the woods, getting feisty inside and out a cabin, and then getting killed.

Luckily, Whedon torques this premise and pushes into unfa-