Add UART ACCEL_READ command for on-demand BMA456 samples.

Expose MessageType 24 with protobuf response (success, x, y, z in raw LSB),
firmware handler with mutex-safe I2C read, goTool `accel` CLI, and docs.

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

goTool/client_api.go

@@ -101,6 +101,25 @@ func decodeClientsPayload(payload []byte) ([]*pb.ClientInfo, error) {
 	return info.GetClients(), nil
 }
 
+func (s *serialPort) readAccel() (*pb.AccelReadResponse, error) {
+	payload, err := s.exchange(byte(pb.MessageType_ACCEL_READ), "ACCEL_READ")
+	if err != nil {
+		return nil, err
+	}
+	var msg pb.UartMessage
+	if err := proto.Unmarshal(payload[1:], &msg); err != nil {
+		return nil, fmt.Errorf("decode: %w", err)
+	}
+	if msg.GetType() != pb.MessageType_ACCEL_READ {
+		return nil, fmt.Errorf("unexpected type %v", msg.GetType())
+	}
+	r := msg.GetAccelReadResponse()
+	if r == nil {
+		return nil, fmt.Errorf("missing accel_read_response")
+	}
+	return r, nil
+}
+
 func (s *serialPort) getVersion() (*pb.VersionResponse, error) {
 	payload, err := s.exchange(byte(pb.MessageType_VERSION), "VERSION")
 	if err != nil {