nRF52840 Power Management - Phase 1 - Boot Low VBAT Voltage Lockout

Added NRF52840 power management core functionality:
- Boot‑voltage lockout
- Initial support for shutdown/reset reason storage and capture (via RESETREAS/GPREGRET2)
- LPCOMP wake (for voltage-driven shutdowns)
- VBUS wake (for voltage-driven shutdowns)
- Per-board shutdown handler for board-specific tasks
- Exposed CLI queries for power‑management status in CommonCLI.cpp
- Added documentation in docs/nrf52_power_management.md.
- Enabled power management support in Xiao nRF52840, RAK4631, Heltec T114 boards

src/helpers/NRF52Board.h

@@ -5,15 +5,58 @@
 
 #if defined(NRF52_PLATFORM)
 
+#ifdef NRF52_POWER_MANAGEMENT
+// Shutdown Reason Codes (stored in GPREGRET before SYSTEMOFF)
+#define SHUTDOWN_REASON_NONE          0x00
+#define SHUTDOWN_REASON_LOW_VOLTAGE   0x4C  // 'L' - Runtime low voltage threshold
+#define SHUTDOWN_REASON_USER          0x55  // 'U' - User requested powerOff()
+#define SHUTDOWN_REASON_BOOT_PROTECT  0x42  // 'B' - Boot voltage protection
+
+// Boards provide this struct with their hardware-specific settings and callbacks.
+struct PowerMgtConfig {
+  // LPCOMP wake configuration (for voltage recovery from SYSTEMOFF)
+  uint8_t lpcomp_ain_channel;       // AIN0-7 for voltage sensing pin
+  uint8_t lpcomp_refsel;            // REFSEL value: 0-6=1/8..7/8, 7=ARef, 8-15=1/16..15/16
+
+  // Boot protection voltage threshold (millivolts)
+  // Set to 0 to disable boot protection
+  uint16_t voltage_bootlock;
+};
+#endif
+
 class NRF52Board : public mesh::MainBoard {
+#ifdef NRF52_POWER_MANAGEMENT
+  void initPowerMgr();
+#endif
+
 protected:
   uint8_t startup_reason;
 
+#ifdef NRF52_POWER_MANAGEMENT
+  uint32_t reset_reason;              // RESETREAS register value
+  uint8_t shutdown_reason;            // GPREGRET value (why we entered last SYSTEMOFF)
+  uint16_t boot_voltage_mv;           // Battery voltage at boot (millivolts)
+
+  bool checkBootVoltage(const PowerMgtConfig* config);
+  void enterSystemOff(uint8_t reason);
+  void configureVoltageWake(uint8_t ain_channel, uint8_t refsel);
+  virtual void initiateShutdown(uint8_t reason);
+#endif
+
 public:
   virtual void begin();
   virtual uint8_t getStartupReason() const override { return startup_reason; }
   virtual float getMCUTemperature() override;
   virtual void reboot() override { NVIC_SystemReset(); }
+
+#ifdef NRF52_POWER_MANAGEMENT
+  bool isExternalPowered() override;
+  uint16_t getBootVoltage() override { return boot_voltage_mv; }
+  virtual uint32_t getResetReason() const override { return reset_reason; }
+  uint8_t getShutdownReason() const override { return shutdown_reason; }
+  const char* getResetReasonString(uint32_t reason) override;
+  const char* getShutdownReasonString(uint8_t reason) override;
+#endif
 };
 
 /*