firmware: add persistent level calibration

tests/test_xiao_sensor_node_contract.py

@@ -44,6 +44,18 @@ def test_gps_auto_detects_common_nmea_baud_rates():
     assert "{9600, 38400, 115200, 4800}" in gps
     assert "passedChecksum() > 0" in gps
 
+def test_level_calibration_is_persistent_and_non_blocking():
+    calibration = source("calibration.cpp")
+    transport = source("rs485_transport.cpp")
+    loop = source("xiao-esp32c6-sensor-node.ino").split("void loop()", 1)[1]
+    assert 'preferences.begin("sensor_cal"' in calibration
+    assert 'putFloat("pitch_zero"' in calibration
+    assert 'putFloat("roll_zero"' in calibration
+    assert 'type == "calibrate_level"' in transport
+    assert 'type == "clear_level_calibration"' in transport
+    assert "updateLevelCalibration" in loop
+    assert "delay(5000)" not in calibration
+
 def test_node_does_not_take_cargo_or_ble_authority():
     combined = "\n".join(p.read_text() for p in NODE.glob("*.*"))
     assert "NimBLE" not in combined and "JBD" not in combined and "server.on(" not in combined