firmware: discard partial RS485 echoes

docs/CHANGELOG.md

@@ -5,6 +5,7 @@
 - Added standalone XIAO ESP32-C6 BNO086/MAX-M10S sensor-node firmware and a versioned RS485 telemetry contract without changing Cargo `/api/v1` or JBD/NimBLE behavior.
 - Added XIAO-side persistent level calibration with stability checking and RS485 start/clear commands; no dashboard behavior changed.
 - Prevented echoed or oversized RS485 frames from starving the XIAO sensor loop.
+- Silently discard malformed partial RS485 echoes instead of emitting recursive `invalid_json` responses.
 
 ## Unreleased
 

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

@@ -89,7 +89,9 @@ void pollRs485Commands(HardwareSerial& serial, const ImuState& imu) {
         StaticJsonDocument<192> request;
         DeserializationError error = deserializeJson(request, commandBuffer);
         commandBuffer = "";
-        if (error) { publishCalibrationResult(serial, "invalid_json"); continue; }
+        // Partial self-echoes and line noise are not commands. Dropping them
+        // silently avoids turning one bad fragment into a response/echo loop.
+        if (error) continue;
         String type = request["type"] | "";
         // Some RS485 boards echo transmitted frames into RX. Ignore our own output.
         if (type == "sensor_status" || type == "calibration_response") continue;

tests/test_xiao_sensor_node_contract.py

@@ -61,6 +61,7 @@ def test_rs485_echo_cannot_starve_sensor_loop():
     assert "bytesRemaining" in transport
     assert "discardCommandUntilNewline" in transport
     assert 'type == "sensor_status"' in transport
+    assert 'publishCalibrationResult(serial, "invalid_json")' not in transport
 
 def test_node_does_not_take_cargo_or_ble_authority():
     combined = "\n".join(p.read_text() for p in NODE.glob("*.*"))