The genuinely and surprisingly scary “Paranormal Activity” is a testament to the culpability of mainstream horror films. What is now considered scary by Hollywood is blood and gore, as evidenced by continuously stale remakes and our good fortune in enduring a sixth “Saw” movie this Halloween. But along comes this Hitchcockian independent feature, shot for a paltry $15,000 and featuring a no-name cast. It is an unsettling experience, with a brilliant story structure utilized to its fullest capacity and savvy enough to not take itself too seriously.