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.

examples/simple_sensor/SensorMesh.cpp

@@ -1316,7 +1316,6 @@ void SensorMesh::begin(FILESYSTEM* fs) {
   updateAdvertTimer();
   updateFloodAdvertTimer();
 
-   board.setAdcMultiplier(_prefs.adc_multiplier);
 
 #if ENV_INCLUDE_GPS == 1
   applyGpsPrefs();