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/MeshCore.h

@@ -56,6 +56,14 @@ public:
   virtual void setGpio(uint32_t values) {}
   virtual uint8_t getStartupReason() const = 0;
   virtual bool startOTAUpdate(const char* id, char reply[]) { return false; }   // not supported
+
+  // Power management interface (boards with power management override these)
+  virtual bool isExternalPowered() { return false; }
+  virtual uint16_t getBootVoltage() { return 0; }
+  virtual uint32_t getResetReason() const { return 0; }
+  virtual const char* getResetReasonString(uint32_t reason) { return "Not available"; }
+  virtual uint8_t getShutdownReason() const { return 0; }
+  virtual const char* getShutdownReasonString(uint8_t reason) { return "Not available"; }
 };
 
 /**