Fix rak_wismeshtag low‑voltage reboot hang after App configuration (#9897)

* Fix TAG low‑voltage reboot hang after App configuration

* nRF52: Move low-VDD System OFF logic to variant hook

* Addressed review

* serialize SAADC access with shared mutex for VDD and battery reads

* raise LPCOMP wake threshold to ensure rising-edge wake

* Trunk fmt

---------

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/Power.cpp

@@ -35,6 +35,11 @@
 #include "nrfx_power.h"
 #endif
 
+#if defined(ARCH_NRF52)
+#include "Nrf52SaadcLock.h"
+#include "concurrency/LockGuard.h"
+#endif
+
 #if defined(DEBUG_HEAP_MQTT) && !MESHTASTIC_EXCLUDE_MQTT
 #include "mqtt/MQTT.h"
 #include "target_specific.h"
@@ -328,6 +333,9 @@ class AnalogBatteryLevel : public HasBatteryLevel
             scaled = esp_adc_cal_raw_to_voltage(raw, adc_characs);
             scaled *= operativeAdcMultiplier;
 #else // block for all other platforms
+#ifdef ARCH_NRF52
+            concurrency::LockGuard saadcGuard(concurrency::nrf52SaadcLock);
+#endif
             for (uint32_t i = 0; i < BATTERY_SENSE_SAMPLES; i++) {
                 raw += analogRead(BATTERY_PIN);
             }