Add persistent battery ADC calibration across boards

Add meter-reference calibration with saved ADC corrections, checked persistence and rollback. Preserve existing board coefficients and nRF52 voltage policy, and merge the Mini Repeater feature gates.

Validation: 69 focused tests, hosted documentation checks, and four firmware builds passed. Mini Repeater retains calibration and its existing filesystem with 376 flash bytes free; earlier Mini Sensor size failures remain documented.

variants/thinknode_m5/ThinknodeM5Board.cpp

@@ -31,7 +31,7 @@ void ThinknodeM5Board::begin() {
     mv /= 8;
 
     analogReadResolution(10);  
-    return static_cast<uint16_t>(mv * ADC_MULTIPLIER );
+    return calibrateBatteryMillivolts(static_cast<uint16_t>(mv * ADC_MULTIPLIER ));
 }
 
   const char* ThinknodeM5Board::getManufacturerName() const {