add Chapter 15 - runtime observer implementation

Chapter15/observer/hal/uart/inc/uart.hpp

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <cstdint>
+#include <span>
+
+namespace hal
+{
+class uart
+{
+  public:
+    virtual void init(std::uint32_t baudrate) = 0;
+    virtual void write(std::span<const char> data) = 0;
+};
+}; // namespace hal