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/max31329.h

@@ -20,6 +20,16 @@ extern "C" {
 
 #define I2C_ADDR 0x68
 
+// MAX31329 register map (the subset this driver touches)
+#define RTC_REG_STATUS 0x00 // Reading clears the alarm flags / releases INTB
+#define RTC_REG_INT_EN 0x01
+#define RTC_REG_CONFIG2 0x04
+#define RTC_REG_SECONDS 0x06 // Start of the 7-byte BCD time block
+#define RTC_REG_ALM2_MIN 0x13 // Start of the 3-byte alarm-2 block
+
+#define RTC_INT_EN_A2IE 0b00000010 // Alarm-2 interrupt enable
+#define RTC_ALM_MASK_BIT 0x80 // "Don't match this field" bit in each alarm register
+
 #define DEC2BCD(n) ((n) + (6 * ((n) / 10)))
 #define BCD2DEC(n) ((n) - (6 * ((n) >> 4)))