JSON WAF Browser + PHP cURL Engine Test v6

Upload/extract so the root directory is:

json-test/

Main pages/files:
- /json-test/ or /json-test/index.php: single-page UI
- /json-test/php-curl-engine.php: JSON engine endpoint used by index.php
- /json-test/api-test.json: static JSON origin endpoint

Changes in v6:
- PHP cURL no longer navigates to a separate result page.
- index.php calls php-curl-engine.php using fetch().
- php-curl-engine.php performs server-side cURL and returns JSON to index.php.
- Browser fetch mode still sends the actual request from the browser and is the mode to use with HAR/DevTools for proving browser challenge flow.
- Single-column layout retained.
