feat(stm32): Add STM32 ADC support to AnalogBatteryLevel (#9369)

Integrate STM32 battery monitoring into AnalogBatteryLevel, supporting
external GPIO ADC pins as well as internal VBAT channel.

Features:
- ADC reading using STM32 LL (Lower Layer) macros supporting external
  ADC channels and internal VBAT channel (AVBAT)
- ADC compensation using STM32 LL macros with factory-calibrated VREFINT
  (AVREF) for accurate voltage measurement
- LFP battery OCV curve for STM32WL using AVBAT (STM32 VDD absolute
  maximum supply voltage 3.9V, direct connection of Li-Po batteries is
  not supported)

Internal VBAT channel implemented in:
- Russell
- RAK3172

In these variants, ADC_MULTIPLIER = (1.01f * 3) = 3.30 as there is a
3:1 internal divider (DS13105 Rev 12 §5.3.21), and a bit of tolerance
as the actual 10% spec leads to readings much too high.


Assisted-by: Claude:sonnet-4-5

Signed-off-by: Andrew Yong <me@ndoo.sg>

variants/stm32/rak3172/variant.h

@@ -16,6 +16,11 @@ Do not expect a working Meshtastic device with this target.
 #define LED_POWER PA0 // Green LED
 #define LED_STATE_ON 1
 
+#define BATTERY_PIN AVBAT
+// ADC_MULTIPLIER: 3.0 = internal 1:3 bridge divider (DS13105§3.18.3)
+// Margin: 1.10 = AVBAT divider tolerance ±10% (Table 82)
+#define ADC_MULTIPLIER (1.01f * 3)
+
 #define RAK3172
 #define SERIAL_PRINT_PORT 1