fix(wio-tracker-l2): keep the last good battery reading when the ADC read fails

A failed ADS1115 read returned 0 mV, so the status bar showed 0 % (and the
low-battery shutdown could see an empty battery) until a reboot. Retry once,
then keep the last good value.

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

variants/wio-tracker-l2/WioTrackerL2Board.cpp

@@ -194,11 +194,18 @@ int16_t WioTrackerL2Board::adsReadRaw() {
   return raw < 0 ? 0 : raw;
 }
 
+// A failed or zero ADS1115 read (the I2C bus busy or the ADC not answering)
+// keeps the last good value instead of reporting 0 mV -- that read as 0 % in
+// the status bar and could trip the low-battery shutdown. Retried once first.
 uint16_t WioTrackerL2Board::getBattMilliVolts() {
   if (!expander_ok) return 0;  // BAT_ADC_EN rail never came up
 
-  int16_t raw = adsReadRaw();
-  if (raw < 0) return 0;
-
-  return (uint16_t)(raw * ADS_MV_PER_LSB);
+  for (int attempt = 0; attempt < 2; attempt++) {
+    int16_t raw = adsReadRaw();
+    if (raw > 0) {
+      batt_mv_last = (uint16_t)(raw * ADS_MV_PER_LSB);
+      return batt_mv_last;
+    }
+  }
+  return batt_mv_last;
 }

variants/wio-tracker-l2/WioTrackerL2Board.h

@@ -93,6 +93,7 @@ private:
   bool expander_ok = false;
   bool aw_ok = false;              // AW35615 USB-C controller responded at probe
   uint8_t wake_btn_baseline = 0;   // idle level of P00, captured at init
+  uint16_t batt_mv_last = 0;       // last good battery reading (a failed read keeps it)
 
   int expReadInputs();   // 16-bit input register pair, -1 on error