Hosted on the backend server. Two iframes load the Unity WebGL client with different
?operator= values; each connects to this host’s WebSocket (ws://…/ws?operator=…).
./scripts/serve-webgl-build.sh in RampJumps (port 8765), then open this page on
http://localhost:8080/multi-operator-test.html. Production: set demo base to your Pages
/demo/index.html URL.