Refactor firmware for clarity; no functional changes

- main.c: file-header comment describing the hardware and operation, logic
  split into named phases (sleep_until_interrupt, wake_peripheral_rails,
  take_counts_snapshot, send_wheel_counts_report, handle_minute_alarm,
  init_all_hardware); shared state renamed to say what it is and made static.
- rfm69.c: reorganized into six labeled sections; cond_1/2/3 and hash scratch
  globals replaced by a reply_acknowledges() helper with clear locals; packet
  layout and every init register write documented.
- LOG() macro (compiled out when DO_UART is off) replaces the #if DO_UART
  blocks that obscured the logic.
- Drivers: file-header comments; named RTC_REG_*/RTC_ALM_MASK_BIT constants;
  EEPROM spool scheme documented; ADC_CHANNEL_BANDGAP named; repeated pin
  if/else helpers collapsed to SET_PIN_TO().
- Removed unused globals/buffers and commented-out code; ran clang-format
  with the project style.

Register writes and radio protocol are byte-identical. Builds clean under
-Wall -Wextra on gnu17 and c23; flash 13028 -> 12830 B, static RAM
1038 -> 999 B.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YVJKatfeMJjAmuH9KYiLuv

avr_code/adc.h

@@ -1,4 +1,4 @@
-/* 
+/*
  * File:   adc.h
  * Author: thebears
  *
@@ -6,17 +6,19 @@
  */
 
 #ifndef ADC_H
-#define	ADC_H
+#define ADC_H
 #include "defines.h"
 #include <avr/io.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <util/delay.h>
 
-#ifdef	__cplusplus
+#ifdef __cplusplus
 extern "C" {
 #endif
 
+// Mux channel 14 is the internal 1.1 V bandgap, used to infer battery voltage.
+#define ADC_CHANNEL_BANDGAP 14
 
 int8_t adc_Initialize(void);
 void adc_Enable(void);
@@ -26,9 +28,8 @@ bool adc_IsConversionDone(void);
 uint16_t adc_GetConversionResult(void);
 uint16_t adc_GetConversion(uint8_t channel);
 
-#ifdef	__cplusplus
+#ifdef __cplusplus
 }
 #endif
 
-#endif	/* ADC_H */
-
+#endif /* ADC_H */