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

README.md

@@ -133,6 +133,8 @@ void receiver_loop() {
 }
 ```
 
+Receiver keeps the SX1276 in continuous RX, re-entering RX after any transmit (ACK or time sync).
+
 **Key receiver functions**:
 ```cpp
 bool lora_receive(LoraPacket &pkt, uint32_t timeout_ms);