When the trailer for Alex Garland’s thriller “Civil War” was released in December, a vast input-output machine creaked into gear. Fans combed and screenshotted the preview for clues about the movie’s dystopia. YouTubers reacted to the brief, tense scenes. Bloggers attempted to sketch out the politics of its imagined second American civil war. A Reddit user isolated and reconstructed a map shown briefly on a television screen — an apparent Rosetta stone to the deep lore fans could look forward to inhaling.