firmware: harden XIAO RS485 telemetry

tests/test_xiao_sensor_node_contract.py

@@ -19,6 +19,9 @@ def test_transport_contract():
     assert "serializeJson(doc, Serial)" in text
     assert 'createNestedObject("nmea")' in text
     assert 'nmea["sentences_passed"]' in text
+    assert 'doc["boot_id"]' in text
+    assert 'doc["frame_sequence"]' in text
+    assert "measureJson(doc)" in text
     for field in ["linear_acceleration_mps2", "angular_velocity_rads", "gravity_mps2", "stability", "hdop", "location_age_ms"]:
         assert field in text
 
@@ -67,6 +70,8 @@ def test_rs485_echo_cannot_starve_sensor_loop():
     assert "discardCommandUntilNewline" in transport
     assert 'type == "sensor_status"' in transport
     assert 'publishCalibrationResult(serial, "invalid_json")' not in transport
+    assert "drainTransmitEcho(serial)" in transport
+    assert transport.index("drainTransmitEcho(serial)") < transport.index("digitalWrite(RS485_DE_PIN, LOW);", transport.index("void sendJson"))
 
 def test_node_does_not_take_cargo_or_ble_authority():
     combined = "\n".join(p.read_text() for p in NODE.glob("*.*"))