Fix WebTransport server implementation and add test client

Server fixes:
- Move H3Connection initialization to ProtocolNegotiated event (matches official aioquic pattern)
- Fix datagram routing to use session_id instead of flow_id
- Add max_datagram_frame_size=65536 to enable QUIC datagrams
- Fix send_datagram() to use keyword arguments
- Add certificate chain handling for Let's Encrypt
- Add no-cache headers to static server

Command-line improvements:
- Move settings from environment variables to argparse
- Add comprehensive CLI arguments with defaults
- Default mode=wt, cert=cert.pem, key=key.pem

Test clients:
- Add test_webtransport_client.py - Python WebTransport client that successfully connects
- Add test_http3.py - Basic HTTP/3 connectivity test

Client updates:
- Auto-configure server URL and certificate hash from /cert-hash.json
- Add ES6 module support

Status:
✅ Python WebTransport client works perfectly
✅ Server properly handles WebTransport connections and datagrams
❌ Chrome fails due to cached QUIC state (QUIC_IETF_GQUIC_ERROR_MISSING)
🔍 Firefox sends packets but fails differently - to be debugged next session

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

server/quic_transport.py

@@ -44,7 +44,7 @@ class GameQuicProtocol(QuicConnectionProtocol):  # type: ignore[misc]
     async def send_datagram(self, data: bytes) -> None:
         logging.debug("QUIC send datagram: %d bytes", len(data))
         self._quic.send_datagram_frame(data)  # type: ignore[attr-defined]
-        await self._loop.run_in_executor(None, self.transmit)  # type: ignore[attr-defined]
+        self.transmit()  # Send queued QUIC packets immediately
 
 
 class QuicWebTransportServer(DatagramServerTransport):
@@ -65,15 +65,20 @@ class QuicWebTransportServer(DatagramServerTransport):
             await proto.send_datagram(data)
 
     async def run(self) -> None:
-        configuration = QuicConfiguration(is_client=False, alpn_protocols=["h3"])
+        configuration = QuicConfiguration(
+            is_client=False,
+            alpn_protocols=["h3"],
+            max_datagram_frame_size=65536  # Enable QUIC datagrams
+        )
         configuration.load_cert_chain(self.certfile, self.keyfile)
 
-        async def _create_protocol(*args, **kwargs):
+        def _create_protocol(*args, **kwargs):
             return GameQuicProtocol(*args, on_datagram=self._on_datagram, peers=self._peers, **kwargs)
 
         logging.info("QUIC datagram server listening on %s:%d", self.host, self.port)
         self._server = await serve(self.host, self.port, configuration=configuration, create_protocol=_create_protocol)
         try:
-            await self._server.wait_closed()
+            # Wait indefinitely until cancelled (e.g., by KeyboardInterrupt or timeout)
+            await asyncio.Event().wait()
         finally:
             self._server.close()