feat(slave_node): Implement initial Modbus RTU server

- Add a basic Modbus RTU server implementation based on Zephyr samples.
- Configure usart1 for Modbus via a board overlay.
- The server initializes and runs, but polling with mbpoll results in a timeout.
- This commit captures a functional but non-working state for further debugging.

software/apps/slave_node/boards/bluepill_f103rb.overlay

@@ -0,0 +1,7 @@
+&usart1 {
+	modbus0 {
+		compatible = "zephyr,modbus-serial";
+		status = "okay";
+	};
+	status = "okay";
+};