e2e runs always test the build that was just made

The test browser keeps Chrome's default profile, disk cache included,
and nginx sends no Cache-Control header. Right after `npm run build` a
run could load the index.html of the previous build and test old code.
It showed up while measuring the scene view pan: a new debug hook
returned NaN although server and bundle already had it.

- signIn() turns the network cache off via
  Network.setCacheDisabled before the first page load.

Tests:
- GM_STEPS=5 (fresh seed): ALL PASSED, 10 checks.
- ./check.sh: CHECK EXIT 0, 2080 tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/lib/harness.mjs

@@ -424,6 +424,9 @@ function makeTools({ cdp, projectId, baseUrl, consoleLog, check, shotPrefix }) {
    */
   const signIn = async () => {
     await cdp.send('Network.enable');
+    // The browser keeps the default profile, disk cache included, and nginx sends no
+    // Cache-Control: a run could test the index.html of a build before the last one.
+    await cdp.send('Network.setCacheDisabled', { cacheDisabled: true });
     await cdp.send('Network.setCookie', {
       name: SESSION_COOKIE,
       value: await sessionToken(baseUrl),