fix(wio-tracker-l2): stop the I2S clock between sounds; GPS and TCXO fixes

The speaker's I2S channel was only disabled when idle, which leaves MCLK
running. Its fractional divider (160 MHz / 39 1/16 = 4.096 MHz) makes a
comb of lines every 256 kHz: one at 869.632 MHz, in the EU narrow mesh
channel, lifted the LoRa noise floor from ~-102 to ~-85 dBm, and others
land near GPS L1 and BeiDou B1. The channel is now deleted and its pins
held low after each sound, and made again for the next one. Measured:
noise floor -102 dBm, outdoor first fix 87 s (was 620 s and more).

Also: TCXO at 3.0 V (the L2's part, as Meshtastic has it); GLONASS
enabled on the L76K after each power-up; begin() on a running receiver
no longer resets it (a reset threw away the acquisition in progress).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

variants/wio-tracker-l2/target.cpp

@@ -23,6 +23,7 @@ void buzzerAmpPower(bool on) { board.setSpeakerAmp(on); }
 #endif
 
 void L2GpsProvider::begin() {
+  if (_started && board.gnssPowered() && !_uart_off) return;   // already running
   board.setGnssPower(true);   // with the power-up reset
   if (_uart_off) {            // back from stop(): the UART again
     Serial1.setRxBufferSize(1024);
@@ -30,6 +31,20 @@ void L2GpsProvider::begin() {
     _uart_off = false;
   }
   MicroNMEALocationProvider::begin();
+  _started = true;
+  _cfg_pending = true;
+  _cfg_rx = rxChars();
+}
+
+// The L76K starts with GPS + BeiDou only; GLONASS as well gives it a third
+// more satellites. Sent once it has booted (NMEA arriving), again after each
+// power-up (not saved in the module).
+void L2GpsProvider::loop() {
+  MicroNMEALocationProvider::loop();
+  if (_cfg_pending && rxChars() - _cfg_rx > 200) {
+    _cfg_pending = false;
+    sendSentence("$PCAS04,7");   // GPS + BeiDou + GLONASS
+  }
 }
 
 void L2GpsProvider::stop() {
@@ -41,7 +56,9 @@ void L2GpsProvider::stop() {
   board.setGnssPower(false);
 }
 
-void L2GpsProvider::reset() { board.gnssReset(); }
+// The sensor manager resets right after every begin(); begin() already did
+// the power-up one, and a second would restart a running receiver.
+void L2GpsProvider::reset() { }
 bool L2GpsProvider::isEnabled() { return board.gnssPowered(); }
 
 bool radio_init() {