Tests three paths to isolate exactly where the "Connecting..." issue is. Log in to /trader/ first.
Bypasses .htaccess, api/index.php, CORS headers. Same as the working diagnostic.
Goes through .htaccess → api/index.php → setCorsHeaders() → router → TraderBrokerController::stream()
Tests the EXACT production path: SharedWorker opens EventSource → /api/trader/broker-connection/stream