Build a 3D arena game as a SINGLE self-contained .html file.
STACK (mandatory):
- Three.js loaded from a CDN (one <script> tag). No other JS libraries, no build step.
- All HTML, CSS, and JS in this one file. It must run by opening it directly in a browser.
CORE SPEC (mandatory — implement all of this exactly):
- A flat ground plane forming a bounded arena. The player cannot leave