In Chloe Okuno’s stylish debut “Watcher,” the title refers not just to one person, but two, when the watched becomes the watcher, the stalker and stalked swapping places throughout the course of this chilly psychological thriller. Working in the vein of ‘70s-style horror, Okuno’s “Watcher” is in dialogue with films like Roman Polanski’s “Repulsion” and “Rosemary’s Baby,” nods to Andrzej Zulawski’s “Possession” with its foreboding European setting, and features a Hitchcock blonde in heroine Julia (Maika Monroe). But those films about vulnerable women caught in voyeuristic traps were all directe…