Native render endpoint (token form)

The engine endpoint the editor itself uses. Hard transport limit: the src token must be at most 16000 characters (about a 16 KB URL); longer requests answer 414 with {ok:false, error:{code:"URL_TOO_LONG"}}. Check url_ok from POST /api/encode; for larger diagrams use POST /api/render (5 MB body) or /api/render/batch.