firmware: add persistent level calibration

docs/CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Added standalone XIAO ESP32-C6 BNO086/MAX-M10S sensor-node firmware and a versioned RS485 telemetry contract without changing Cargo `/api/v1` or JBD/NimBLE behavior.
+- Added XIAO-side persistent level calibration with stability checking and RS485 start/clear commands; no dashboard behavior changed.
 
 ## Unreleased
 

docs/SENSOR_NODE_PROTOCOL.md

@@ -16,3 +16,9 @@ The expansion-board pins are D2 driver enable, D4 TX, and D5 RX. Only physical b
 Consumers must check `valid` or `fix` before displaying measurements and ignore unknown fields. The native breakout coordinate frame is reported initially; installation-specific axis remapping should follow physical calibration.
 
 This link does not replace dashboard-to-Cargo WiFi/HTTP. A future dashboard receiver may display this telemetry without moving relay, BMS, alarm, or configuration authority away from Cargo.
+
+## Level calibration commands
+
+The receiver may send `{"type":"calibrate_level"}` followed by a newline. The node immediately responds with `calibration_response` result `started`, then samples the IMU for five seconds. If pitch or roll moves by more than two degrees, it responds with `vehicle_moved` and retains the previous calibration. On success it saves averaged pitch and roll offsets in NVS and responds with `complete`.
+
+Send `{"type":"clear_level_calibration"}` to erase the saved offsets. Telemetry retains native `imu.pitch_degrees` and `imu.roll_degrees` and adds `imu.level_calibration` containing status, offsets, and calibrated pitch/roll. Level calibration intentionally does not alter yaw.