'auto' (default): WebTransport when it is available AND connects;
otherwise WebSocket. A WebTransport connect failure (most commonly
UDP/QUIC blocked by a school or corporate network — the API exists
but can never reach the relay) falls through to WebSocket instead of
erroring. Auth failures (401/403) surface immediately, never fall back.
'webtransport' / 'websocket': force one transport; connect errors
surface directly.
Transport selection policy.
'auto'(default): WebTransport when it is available AND connects; otherwise WebSocket. A WebTransport connect failure (most commonly UDP/QUIC blocked by a school or corporate network — the API exists but can never reach the relay) falls through to WebSocket instead of erroring. Auth failures (401/403) surface immediately, never fall back.'webtransport'/'websocket': force one transport; connect errors surface directly.