Challenge 1: A Circle With an area Method — Possible Solution
====================================================================

struct Circle {
    radius: f64,
}

impl Circle {
    fn area(&self) -> f64 {
        3.14159 * self.radius * self.radius
    }
}

fn main() {
    let c = Circle { radius: 4.0 };
    println!("Area: {}", c.area());
}

WHY THIS WORKS AS AN ANSWER
------------------------------
struct Circle { radius: f64 } defines exactly the one required field,
following the same shape as this chapter's Rectangle example.

The area method takes &self — an immutable borrowed reference to the
instance, per Chapter 4's borrowing — giving it read access to
self.radius without taking ownership. The area calculation itself
(pi * r * r) is standard geometry, using the field via self.radius.

c.area() calls the method using dot syntax, the same instance.method()
pattern this chapter established, and works because area is a METHOD
(takes self), not an associated function.