Challenge 1: A geometry Namespace With area() — Possible Solution
====================================================================

#include <iostream>

namespace geometry {
    double area(double radius) {
        return 3.14159 * radius * radius;
    }
}

int main() {
    std::cout << geometry::area(5.0) << std::endl;
    return 0;
}

Output:

    78.5397

WHY THIS WORKS AS AN ANSWER
------------------------------
area() is declared inside the geometry namespace, so it's only
reachable via its fully qualified name geometry::area(...) -- exactly
the syntax the chapter's own myapp::process() example demonstrates --
rather than existing as a plain, globally-visible area() that could
collide with an unrelated function of the same name elsewhere in a
larger project.