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

src/lora_transport.cpp

@@ -118,6 +118,10 @@ void lora_sleep() {
   LoRa.sleep();
 }
 
+void lora_receive_continuous() {
+  LoRa.receive();
+}
+
 bool lora_receive_window(LoraPacket &pkt, uint32_t timeout_ms) {
   if (timeout_ms == 0) {
     return false;