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_m3/ThinkNodeM3Board.cpp

@@ -24,5 +24,5 @@ uint16_t ThinkNodeM3Board::getBattMilliVolts() {
   adcvalue = analogRead(PIN_VBAT_READ);
   // Convert the raw value to compensated mv, taking the resistor-
   // divider into account (providing the actual LIPO voltage)
-  return (uint16_t)((float)adcvalue * ADC_FACTOR);
+  return calibrateBatteryMillivolts((uint16_t)((float)adcvalue * ADC_FACTOR));
 }