fix: rtcp listener tracks server-negotiated bind address for prefix fallback

The rtcp listener now stores the address returned by Router.Bind() separately
from the requested bind address. This allows callers to distinguish between
the requested prefix (may be rejected by the relay) and the actual server-
assigned address (used for the entrypoint URL).

listener/rtcp/listener.go

@@ -24,13 +24,14 @@ func init() {
 }
 
 type rtcpListener struct {
-	laddr   net.Addr
-	ln      net.Listener
-	logger  logger.Logger
-	closed  chan struct{}
-	md      metadata
-	options listener.Options
-	mu      sync.Mutex
+	laddr    net.Addr // requested bind address (immutable after Init)
+	bindAddr net.Addr // server-negotiated address from the last successful Bind (guarded by mu)
+	ln       net.Listener
+	logger   logger.Logger
+	closed   chan struct{}
+	md       metadata
+	options  listener.Options
+	mu       sync.Mutex
 }
 
 func NewListener(opts ...listener.Option) listener.Listener {
@@ -81,6 +82,8 @@ func (l *rtcpListener) Accept() (conn net.Conn, err error) {
 			return nil, listener.NewBindError(err)
 		}
 
+		l.setBindAddr(ln.Addr())
+
 		ln = metrics.WrapListener(l.options.Service, ln)
 		ln = stats.WrapListener(ln, l.options.Stats)
 		ln = admission.WrapListener(l.options.Service, l.options.Admission, ln)
@@ -116,7 +119,14 @@ func (l *rtcpListener) Accept() (conn net.Conn, err error) {
 	return
 }
 
+// Addr returns the server-negotiated bind address once the tunnel bind has
+// completed, falling back to the requested address before that.
 func (l *rtcpListener) Addr() net.Addr {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	if l.bindAddr != nil {
+		return l.bindAddr
+	}
 	return l.laddr
 }
 
@@ -133,6 +143,14 @@ func (l *rtcpListener) Close() error {
 	return nil
 }
 
+func (l *rtcpListener) setBindAddr(addr net.Addr) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	if addr != nil {
+		l.bindAddr = addr
+	}
+}
+
 func (l *rtcpListener) setListener(ln net.Listener) {
 	l.mu.Lock()
 	defer l.mu.Unlock()