refactor: adopt layered C++ project layout

src/app/main.cpp

@@ -0,0 +1,20 @@
+#include <project/sample_library.hpp>
+
+#include <iostream>
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    std::cin.tie(nullptr);
+
+    const project::Greeter greeter{"Welcome"};
+    std::cout << greeter.greet("Coding World") << '\n';
+
+    auto sequence = project::generate_sequence(5);
+    std::cout << "Sample sequence:";
+    for (int value : sequence) {
+        std::cout << ' ' << value;
+    }
+    std::cout << '\n';
+
+    return 0;
+}

src/lib/sample_library.cpp

@@ -0,0 +1,21 @@
+#include <project/sample_library.hpp>
+
+#include <numeric>
+
+namespace project {
+
+std::string Greeter::greet(const std::string &name) const {
+    return base_message_ + ", " + name + "!";
+}
+
+std::vector<int> generate_sequence(int limit) {
+    if (limit < 0) {
+        limit = 0;
+    }
+
+    std::vector<int> result(static_cast<std::size_t>(limit));
+    std::iota(result.begin(), result.end(), 0);
+    return result;
+}
+
+}  // namespace project

src/main.cpp

@@ -1,14 +0,0 @@
-#include <iostream>
-#include <vector>
-#include <string>
-#include <algorithm>
-#include <numeric>
-
-int main() {
-    std::ios::sync_with_stdio(false);
-    std::cin.tie(nullptr);
-
-    // TODO: Add program logic here.
-
-    return 0;
-}