refactor: adopt layered C++ project layout

include/project/sample_library.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace project {
+
+// Greeter builds a greeting message for a given name.
+class Greeter {
+public:
+    explicit Greeter(std::string base = "Hello") : base_message_(std::move(base)) {}
+
+    [[nodiscard]] std::string greet(const std::string &name) const;
+
+private:
+    std::string base_message_;
+};
+
+// generate_sequence returns numbers from 0 up to limit (exclusive).
+[[nodiscard]] std::vector<int> generate_sequence(int limit);
+
+}  // namespace project