Rock Band developer Harmonix has been venturing out into new territory since the music-instrument-game craze fizzled, jettisoning a wall of plastic guitars at your local Best Buy. Announced yesterday, Chroma is the most unbridled departure yet: a rhythm-game and an FPS with a Tron-like battle arena and dubstep. Yes, that is a sentence you just read.  “How the fuck does that work?” you may rightly ask. Well, guns and characters are dedicated to different music genres, say hip-hop or free jazz. They shoot sequences of musical notes, presumably based on which buttons you press. Ultimately it comes down to killing…