Two Long Island teens, heading home from a birthday party, were killed on the Southern State Parkway yesterday when an SUV traveling in the opposite direction careened into a median, launched into the air and landed on top of their car. Promising student Luis Alfaro, 19, of Brentwood, who had borrowed his aunt's Chevrolet Cavalier to go to the party in Freeport with his neighbor, Sammy Mena, 17, were killed instantly by the smash in the eastbound lane near Exit 42 in Bay Shore, authorities said. The two occupants of the SUV Ford Explorer — which then rolled over and...