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_m8/ThinkNodeM8Board.cpp

@@ -33,6 +33,6 @@ uint16_t ThinkNodeM8Board::getBattMilliVolts() {
   // Convert the raw value to compensated mv, taking the resistor-
   // divider into account (providing the actual LIPO voltage)
   digitalWrite(ADC_EN, LOW);
-  return (uint16_t)((float)adcvalue * REAL_VBAT_MV_PER_LSB);
+  return calibrateBatteryMillivolts((uint16_t)((float)adcvalue * REAL_VBAT_MV_PER_LSB));
 }
 #endif