Generate a random 256-bit token when spawning a proxy process, pass
it as a CLI argument, and return it to the client in the stream start
response. The proxy validates the token on WebTransport connect and
consumes it after first use, preventing replay. A wrong token attempt
also consumes the token for security. Includes 5 unit tests for token
validation logic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

@@ -80,1 +80,4 @@
 
+/// Validate a provided token against the stored token. Consumes the token on success (single-use).
+/// Returns Ok(()) if valid, Err with description if invalid or already consumed.
+pub async fn validate_stream_token(proxy: &Proxy, provided: &str) -> std::result::Result<(), String> {