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_m2/ThinknodeM2Board.cpp

@@ -22,7 +22,7 @@ uint16_t ThinknodeM2Board::getBattMilliVolts() {
   mv /= 8;
 
   analogReadResolution(10);
-  return static_cast<uint16_t>(mv * ADC_MULTIPLIER);
+  return calibrateBatteryMillivolts(static_cast<uint16_t>(mv * ADC_MULTIPLIER));
 }
 
 const char *ThinknodeM2Board::getManufacturerName() const {