If “Greyson Brust” is as killer as Harry Warden, then “Letha Mondragon” is easily as inviolable as Laurie Strode, as Sidney Prescott, both of whom dress conservatively, neither of whom would ever bleach her hair with stolen peroxide in a hospital sink, then dye it electric blue. No, Jade will never be any kind of final girl, she knows, and has known for years.