Add accelerometer deadzone via UART and ESP-NOW.

Filter BMA456 logs by configurable LSB threshold; master can set deadzone
for local sensor or slaves using ACCEL_DEADZONE (UART) and ESP-NOW broadcast
until unicast delivery is restored.

Co-authored-by: Cursor <cursoragent@cursor.com>

goTool/serialport.go

@@ -38,6 +38,32 @@ func (s *serialPort) Close() error {
 	return s.port.Close()
 }
 
+func (s *serialPort) exchangePayload(payload []byte, cmdName string) ([]byte, error) {
+	if len(payload) == 0 {
+		return nil, fmt.Errorf("empty payload")
+	}
+	frame, err := uartframe.EncodeFrame(payload)
+	if err != nil {
+		return nil, fmt.Errorf("encode frame: %w", err)
+	}
+
+	log.Printf("sending %s command (%d bytes): % x", cmdName, len(frame), frame)
+	if _, err := s.port.Write(frame); err != nil {
+		return nil, fmt.Errorf("write: %w", err)
+	}
+
+	respPayload, err := uartframe.ReadFrame(s.port, nil)
+	if err != nil {
+		return nil, fmt.Errorf("read response: %w", err)
+	}
+
+	log.Printf("response payload (%d bytes): % x", len(respPayload), respPayload)
+	if len(respPayload) == 0 {
+		return nil, fmt.Errorf("empty response payload")
+	}
+	return respPayload, nil
+}
+
 func (s *serialPort) exchange(cmdID byte, cmdName string) ([]byte, error) {
 	frame, err := uartframe.EncodeFrame([]byte{cmdID})
 	if err != nil {