Keep receiver LoRa in continuous RX

- 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

include/lora_transport.h

@@ -21,5 +21,6 @@ bool lora_send(const LoraPacket &pkt);
 bool lora_receive(LoraPacket &pkt, uint32_t timeout_ms);
 void lora_idle();
 void lora_sleep();
+void lora_receive_continuous();
 bool lora_receive_window(LoraPacket &pkt, uint32_t timeout_ms);
 uint32_t lora_airtime_ms(size_t packet_len);