fix: make router socket non-blocking to prevent poll loop stalls

sketch/SerialTransport.h

@@ -0,0 +1,51 @@
+/*
+    This file is part of the Arduino_RPClite library.
+
+    Copyright (c) 2025 Arduino SA
+
+    This Source Code Form is subject to the terms of the Mozilla Public
+    License, v. 2.0. If a copy of the MPL was not distributed with this
+    file, You can obtain one at http://mozilla.org/MPL/2.0/.
+    
+*/
+
+#ifndef SERIALTRANSPORT_H
+#define SERIALTRANSPORT_H
+#include "transport.h"
+
+class SerialTransport final : public ITransport {
+
+    Stream* _stream;
+
+    public:
+
+        explicit SerialTransport(Stream* stream): _stream(stream){}
+
+        explicit SerialTransport(Stream& stream): _stream(&stream){}
+
+        bool available() override {
+            return _stream->available();
+        }
+
+        size_t write(const uint8_t* data, size_t size) override {
+            _stream->write(data, size);
+            return size;
+        }
+
+        size_t read(uint8_t* buffer, size_t size) override {
+            _stream->setTimeout(0);
+            return _stream->readBytes(buffer, size);
+        }
+
+        size_t read_byte(uint8_t& r) override {
+            uint8_t b[1];
+            if (read(b, 1) != 1){
+                return 0;
+            };
+            r = b[0];
+            return 1;
+        }
+
+};
+
+#endif  //SERIALTRANSPORT_H