firmware: expose GPS NMEA diagnostics

tests/test_xiao_sensor_node_contract.py

@@ -17,6 +17,8 @@ def test_transport_contract():
         assert token in text
     assert "USB_TELEMETRY_ENABLED" in text
     assert "serializeJson(doc, Serial)" in text
+    assert 'createNestedObject("nmea")' in text
+    assert 'nmea["sentences_passed"]' in text
 
 def test_loop_is_non_blocking():
     loop = source("xiao-esp32c6-sensor-node.ino").split("void loop()", 1)[1]