- Add lora_receive_continuous() helper and use it after init and TX (ACK/time sync)
- Ensure receiver returns to RX immediately after lora_send
- Document continuous RX behavior in README
- Add LoRa idle/sleep/receive-window helpers and use short RX windows for ACK/time sync
- Schedule sender time-sync windows (fast/slow) and track RX vs sleep time in debug
- Lower sender power (80 MHz CPU, WiFi/BT off, reduced ADC sampling, unused pins pulldown)
- Make SERIAL_DEBUG_MODE a build flag, add prod envs with debug off, and document changes