Add MASQUE/CONNECT-UDP support (RFC 9298)

Implement UDP tunneling over HTTP/3 using HTTP Datagrams (RFC 9297):
- Add masque handler for server-side CONNECT-UDP
- Add masque connector and h3-masque dialer for client-side
- Add enableDatagrams option to HTTP/3 listener
- Add shared utilities for datagram connections and path parsing

Resource management and connection caching:
- Add deferred stream cleanup in connector on error paths
- Add IsClosed() and Close() methods to Client for proper session management
- Clean up stale cached clients in dialer before reuse
- Close underlying stream when DatagramConn is closed
- Move RequestStream opening from connector to dialer to enable dead
  connection detection and cache invalidation (follows QUIC dialer pattern)

listener/http3/listener.go

@@ -75,8 +75,10 @@ func (l *http3Listener) Init(md md.Metadata) (err error) {
 			},
 			MaxIncomingStreams: int64(l.md.maxStreams),
 			Allow0RTT:          true,
+			EnableDatagrams:    l.md.enableDatagrams,
 		},
-		Handler: http.HandlerFunc(l.handleFunc),
+		EnableDatagrams: l.md.enableDatagrams,
+		Handler:         http.HandlerFunc(l.handleFunc),
 	}
 
 	ln, err := quic.ListenAddrEarly(addr, http3.ConfigureTLSConfig(l.server.TLSConfig), l.server.QUICConfig.Clone())

listener/http3/metadata.go

@@ -19,6 +19,7 @@ type metadata struct {
 	maxIdleTimeout   time.Duration
 	handshakeTimeout time.Duration
 	maxStreams       int
+	enableDatagrams  bool
 }
 
 func (l *http3Listener) parseMetadata(md mdata.Metadata) (err error) {
@@ -46,6 +47,7 @@ func (l *http3Listener) parseMetadata(md mdata.Metadata) (err error) {
 	l.md.handshakeTimeout = mdutil.GetDuration(md, handshakeTimeout)
 	l.md.maxIdleTimeout = mdutil.GetDuration(md, maxIdleTimeout)
 	l.md.maxStreams = mdutil.GetInt(md, maxStreams)
+	l.md.enableDatagrams = mdutil.GetBool(md, "enableDatagrams")
 
 	return
 }