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>

src/helpers/ui/buzzer.cpp

@@ -426,11 +426,22 @@ static bool i2sInstall() {
 }
 static void i2sUninstall() { i2s_channel_disable(s_tx); i2s_del_channel(s_tx); s_tx = nullptr; }
 static void i2sWrite(const void* src, size_t n, uint32_t ms) {
+  if (!s_tx) return;
   size_t w;
   i2s_channel_write(s_tx, src, n, &w, ms);
 }
-static void i2sStop()    { i2s_channel_disable(s_tx); }   // clocks off
-static void i2sRestart() { i2s_channel_enable(s_tx); }    // auto-clear left the buffers silent
+// Clocks off between sounds: a disabled channel still drives MCLK, and its
+// fractional divider (160 MHz / 39 1/16) makes a comb of lines every 256 kHz
+// right up the LoRa band -- 869.632 MHz sat in the EU narrow channel, ~15 dB
+// over the Wio Tracker L2's noise floor. So the channel goes, the pins held
+// low, and a sound makes it again.
+static void i2sStop() {
+  if (!s_tx) return;
+  i2sUninstall();
+  const int pins[] = { PIN_I2S_MCLK, PIN_I2S_BCK, PIN_I2S_WS, PIN_I2S_DOUT };
+  for (int p : pins) { gpio_reset_pin((gpio_num_t)p); pinMode(p, OUTPUT); digitalWrite(p, LOW); }
+}
+static void i2sRestart() { if (!s_tx) i2sInstall(); }
 #else
 static const i2s_port_t I2S_PORT = I2S_NUM_0;
 static bool i2sInstall() {