firmware: harden XIAO RS485 telemetry

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

@@ -1,4 +1,5 @@
 #include <ArduinoJson.h>
+#include <esp_system.h>
 #include "rs485_transport.h"
 #include "sensor_node_config.h"
 #include "calibration.h"
@@ -6,9 +7,28 @@
 namespace {
 String commandBuffer;
 bool discardCommandUntilNewline = false;
+uint32_t bootId = 0;
+uint32_t frameSequence = 0;
+
+void drainTransmitEcho(HardwareSerial& serial) {
+    while (serial.available()) serial.read();
+}
+
 void sendJson(HardwareSerial& serial, JsonDocument& doc) {
-    digitalWrite(RS485_DE_PIN, HIGH); delayMicroseconds(20);
+    if (doc.overflowed() || measureJson(doc) + 1 > RS485_MAX_FRAME_BYTES) {
+#if USB_TELEMETRY_ENABLED
+        Serial.println("{\"type\":\"rs485_error\",\"error\":\"frame_too_large\"}");
+#endif
+        return;
+    }
+
+    digitalWrite(RS485_DE_PIN, HIGH);
+    delayMicroseconds(RS485_DRIVER_ENABLE_SETTLE_US);
     serializeJson(doc, serial); serial.write('\n'); serial.flush();
+    // This expansion board can echo TX into RX. At flush completion the
+    // entire transmitted line has left the UART, so discard that local echo
+    // before returning the transceiver to receive mode.
+    drainTransmitEcho(serial);
     digitalWrite(RS485_DE_PIN, LOW);
 #if USB_TELEMETRY_ENABLED
     serializeJson(doc, Serial); Serial.println();
@@ -18,13 +38,17 @@ void sendJson(HardwareSerial& serial, JsonDocument& doc) {
 
 void initRs485(HardwareSerial& serial) {
     pinMode(RS485_DE_PIN, OUTPUT); digitalWrite(RS485_DE_PIN, LOW);
+    serial.setRxBufferSize(RS485_RX_BUFFER_BYTES);
     serial.begin(RS485_BAUD, SERIAL_8N1, RS485_RX_PIN, RS485_TX_PIN);
+    bootId = esp_random();
 }
 
 void publishSensorStatus(HardwareSerial& serial, const ImuState& imu, const GpsState& gps) {
     StaticJsonDocument<2048> doc;
     doc["type"] = "sensor_status"; doc["schema_version"] = SENSOR_NODE_SCHEMA_VERSION;
     doc["node_id"] = SENSOR_NODE_NAME; doc["firmware_version"] = SENSOR_NODE_FIRMWARE_VERSION;
+    doc["boot_id"] = bootId;
+    doc["frame_sequence"] = frameSequence++;
     doc["uptime_ms"] = millis();
     JsonObject i = doc.createNestedObject("imu");
     i["online"] = imu.online; i["valid"] = imu.valid && millis() - imu.lastUpdateMs <= SENSOR_STALE_MS;