firmware: add persistent level calibration

firmware/xiao-esp32c6-sensor-node/xiao-esp32c6-sensor-node.ino

@@ -3,6 +3,7 @@
 #include "imu.h"
 #include "rs485_transport.h"
 #include "sensor_node_config.h"
+#include "calibration.h"
 
 HardwareSerial gpsSerial(0), rs485Serial(1);
 ImuState imuState;
@@ -13,16 +14,21 @@ void setup() {
     Serial.begin(115200); delay(250);
     Serial.println("XIAO ESP32-C6 sensor node booting");
     initGps(gpsSerial); initRs485(rs485Serial);
+    initLevelCalibration();
     imuState.online = initImu();
     Serial.println(imuState.online ? "BNO086 ready" : "BNO086 not detected");
 }
 
 void loop() {
     updateGps(gpsSerial, gpsState); updateImu(imuState);
+    pollRs485Commands(rs485Serial, imuState);
+    String calibrationResult;
+    if (updateLevelCalibration(imuState, calibrationResult)) {
+        publishCalibrationResult(rs485Serial, calibrationResult);
+    }
     unsigned long now = millis();
     if (now - lastTelemetryMs >= TELEMETRY_INTERVAL_MS) {
         publishSensorStatus(rs485Serial, imuState, gpsState); lastTelemetryMs = now;
     }
     delay(1);
 }
-