Render a URL in a real headless browser (JavaScript executed) and return its article content with the clutter stripped — same cleaning + output formats as /api/url/clean, but for client-rendered / SPA pages whose content only appears after JS runs (where a raw HTTP fetch returns an empty shell). `format`: markdown (default), text, both (JSON envelope), html (self-contained reader page, raw text/html), or pdf (typeset reading doc, raw application/pdf). Optional `waitUntil` (load|domcontentloaded|networkidle0|networkidle2, default networkidle2) and `timeoutMs` (1000-15000, default 12000) control how long to let JS settle. Use /api/url/clean instead for server-rendered pages — it is faster and cheaper; only reach for render when JS rendering is required.