fix(handler/serial): record-after-write ordering, nil Router guards, consistent ReadTimeout

- conn.go: Write() now writes first then records b[:n] (not len(b)),
  matching the Read() pattern and preventing phantom data on failures.
- handler.go: add nil Router check before h.options.Router.Dial()
  in Handle's non-hop path to return a clear error instead of panicking.
- handler.go: guard h.options.Router against nil in forwardSerial;
  restructure if/else so ReadTimeout is applied in both the direct
  OpenPort path and when the Router has no chain configured.

handler/serial/handler.go

@@ -93,6 +93,11 @@ func (h *serialHandler) Handle(ctx context.Context, conn net.Conn, opts ...handl
 		return h.forwardSerial(ctx, conn, target, log)
 	}
 
+	if h.options.Router == nil {
+		err := errors.New("router not available")
+		log.Error(err)
+		return err
+	}
 	cc, err := h.options.Router.Dial(ctx, "tcp", "@")
 	if err != nil {
 		log.Error(err)
@@ -118,9 +123,12 @@ func (h *serialHandler) forwardSerial(ctx context.Context, conn net.Conn, target
 	cfg := serial.ParseConfigFromAddr(conn.LocalAddr().String())
 	cfg.Name = target.Addr
 
-	if opts := h.options.Router.Options(); opts != nil && opts.Chain != nil {
-		port, err = h.options.Router.Dial(ctx, "serial", serial.AddrFromConfig(cfg))
-	} else {
+	if h.options.Router != nil {
+		if opts := h.options.Router.Options(); opts != nil && opts.Chain != nil {
+			port, err = h.options.Router.Dial(ctx, "serial", serial.AddrFromConfig(cfg))
+		}
+	}
+	if port == nil && err == nil {
 		cfg.ReadTimeout = h.md.timeout
 		port, err = serial.OpenPort(cfg)
 	}