Class SemanticRelayClient

Open the WebTransport connection, read the session:info handshake frame, and start the datagram read loop. Returns the parsed SessionInfo once the server confirms join. Retries transient failures with backoff; surfaces auth failures immediately.

Close the connection, drop subscribers, and release the writer. Idempotent.

Fire-and-forget publish. The 4-byte sender session ID is stamped by the server, so the wire payload is just the encoded envelope.

Subscribe to a topic. Returns an unsubscribe function. Multiple subscribers per topic are supported and dispatched in registration order.