Make browser extension generation deterministic

[Copilot]

The browser GitHub Actions job was failing even when the browser engine checks passed. The failure came from browser/build-extension.mjs regenerating extension/engine.js with a day-dependent header, causing a diff on every later run.

• Root cause

  • extension/engine.js included a generated Built: YYYY-MM-DD line.
  • The workflow intentionally rebuilds the extension engine and fails if the committed output changes, so the timestamp created daily false-positive drift.

• Change

  • Remove the date stamp from the generated header in browser/build-extension.mjs.
  • Regenerate extension/engine.js so the committed artifact matches the stable generator output.

• Effect

  • Re-running the browser extension build no longer changes extension/engine.js unless the actual engine source changes.
  • The workflow still preserves its sync check between browser/mechcheck.html and the extension copy.

const header = `/* GENERATED FILE -- do not edit.
 *
 * Built from browser/mechcheck.html by browser/build-extension.mjs, so the
 * extension and the standalone page run byte-for-byte the same checks. Edit
 * mechcheck.html and re-run the build.
 */
`;