firmware: auto-detect GPS UART baud

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

@@ -3,12 +3,48 @@
 #include "sensor_node_config.h"
 namespace { TinyGPSPlus parser; }
 
-void initGps(HardwareSerial& serial) { serial.begin(GPS_BAUD, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN); }
+namespace {
+uint32_t activeBaud = GPS_FALLBACK_BAUD;
+
+bool baudHasValidNmea(HardwareSerial& serial, uint32_t baud) {
+    TinyGPSPlus probe;
+    serial.end();
+    serial.begin(baud, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN);
+    unsigned long started = millis();
+    while (millis() - started < GPS_BAUD_DETECT_MS) {
+        while (serial.available()) probe.encode(serial.read());
+        if (probe.passedChecksum() > 0) return true;
+        delay(1);
+    }
+    return false;
+}
+}
+
+void initGps(HardwareSerial& serial) {
+    const uint32_t candidates[] = {9600, 38400, 115200, 4800};
+    Serial.println("Detecting GPS UART baud...");
+    for (uint32_t baud : candidates) {
+        Serial.print("Trying GPS baud ");
+        Serial.println(baud);
+        if (!baudHasValidNmea(serial, baud)) continue;
+        activeBaud = baud;
+        Serial.print("Valid GPS NMEA detected at ");
+        Serial.println(activeBaud);
+        return;
+    }
+
+    activeBaud = GPS_FALLBACK_BAUD;
+    serial.end();
+    serial.begin(activeBaud, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN);
+    Serial.println("No valid GPS NMEA detected; falling back to 9600 baud");
+}
+
 void updateGps(HardwareSerial& serial, GpsState& s) {
     while (serial.available()) { s.lastByteMs = millis(); parser.encode(serial.read()); }
     s.charactersProcessed = parser.charsProcessed();
     s.sentencesPassed = parser.passedChecksum();
     s.sentencesFailed = parser.failedChecksum();
+    s.baud = activeBaud;
     s.online = s.lastByteMs && millis() - s.lastByteMs <= SENSOR_STALE_MS;
     s.fix = parser.location.isValid() && parser.location.age() <= SENSOR_STALE_MS;
     s.satellites = parser.satellites.isValid() ? parser.satellites.value() : 0;