firmware: mirror sensor telemetry to USB

firmware/xiao-esp32c6-sensor-node/README.md

@@ -14,3 +14,5 @@ Power both sensors from 3.3V with shared ground. Required Arduino libraries are
 At startup the firmware scans D7/D8 and accepts either valid BNO08x address, `0x4A` or `0x4B`. If neither appears in the scan, verify 3.3V/GND, swap-check SDA and SCL, and confirm the breakout's PS0/PS1 jumpers are configured for I2C.
 
 The node sends `sensor_status` at 115200 baud every 200 ms. See `docs/SENSOR_NODE_PROTOCOL.md`. The initial firmware is transmit-only; add a dashboard receiver after bench-testing the node, RS485 polarity, and sensor axes.
+
+During bring-up, `USB_TELEMETRY_ENABLED` mirrors each RS485 JSON frame to the Arduino Serial Monitor at 115200 baud. Set it to `0` in `sensor_node_config.h` when continuous USB output is no longer wanted.

firmware/xiao-esp32c6-sensor-node/rs485_transport.cpp

@@ -26,5 +26,9 @@ void publishSensorStatus(HardwareSerial& serial, const ImuState& imu, const GpsS
     digitalWrite(RS485_DE_PIN, HIGH); delayMicroseconds(20);
     serializeJson(doc, serial); serial.write('\n'); serial.flush();
     digitalWrite(RS485_DE_PIN, LOW);
-}
 
+#if USB_TELEMETRY_ENABLED
+    serializeJson(doc, Serial);
+    Serial.println();
+#endif
+}

firmware/xiao-esp32c6-sensor-node/sensor_node_config.h

@@ -19,3 +19,4 @@
 #define IMU_REPORT_INTERVAL_US 20000
 #define TELEMETRY_INTERVAL_MS 200
 #define SENSOR_STALE_MS 2000
+#define USB_TELEMETRY_ENABLED 1

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]