It has taken me eight months and three viewings to wrap my mind around “Possessor.” From the beginning, I recognized Brandon Cronenberg’s brilliant sci-fi puzzle to be more than just another bracingly extreme psychological thriller, but I wasn’t sure what to make of the violence. Like a cold metal probe to the cortex, you can’t un-think certain images once they take root there. “Possessor” is by far the most disturbing film I’ve ever seen at Sundance, for reasons that, if I were to list them, would not only spoil the experience, but send a certain contingent of shock-horror fans rushing to see what all the fuss was about.