Challenge 2: Calling C's sqrt via FFI — Possible Solution
====================================================================

extern "C" {
    fn sqrt(x: f64) -> f64;
}

fn main() {
    let result = unsafe { sqrt(16.0) };
    println!("{}", result); // 4
}

WHY THIS WORKS AS AN ANSWER
------------------------------
The extern "C" block declares sqrt's signature to Rust WITHOUT
providing any Rust implementation for it — this tells the compiler
"a function matching this exact signature exists somewhere, compiled
from C, and will be linked in" — exactly this chapter's pattern for
declaring a foreign function, following the abs example directly.

Calling sqrt(16.0) is wrapped in an unsafe block because ALL calls to
extern "C" functions require it, per this chapter's explanation: the
Rust compiler has no way to verify a foreign, non-Rust function's
internal safety guarantees — it simply trusts the declared signature
and the caller's own judgment that using it is sound.

Since sqrt is a standard C math library function widely available on
the linking platform (part of libm), no special build configuration
is needed for this to link successfully in most environments — a
concrete, minimal example of exactly the "interop with existing C
libraries" use case this chapter named as FFI's genuinely practical
purpose.