Executable specification
The browser's geolocation as a fix. The only part of the real client that is arithmetic rather than wiring, and the part nobody can check by looking: a speed derived the wrong way is a TTL counting down to nothing, and it would look entirely plausible on screen.
The same assertions tools/run-js-tests.mjs runs in the Maven build, imported from receiver-test.js rather than copied, so the page and the build cannot disagree.