firmware: mirror sensor telemetry to USB

tests/test_xiao_sensor_node_contract.py

@@ -15,6 +15,8 @@ def test_transport_contract():
     text = source("rs485_transport.cpp")
     for token in ['doc["type"] = "sensor_status"', 'doc["schema_version"]', 'createNestedObject("imu")', 'createNestedObject("gps")', "serial.write('\\n')", "serial.flush()"]:
         assert token in text
+    assert "USB_TELEMETRY_ENABLED" in text
+    assert "serializeJson(doc, Serial)" in text
 
 def test_loop_is_non_blocking():
     loop = source("xiao-esp32c6-sensor-node.ino").split("void loop()", 1)[1]