Challenge 3: Double Shadowing — Possible Solution
====================================================================

fn main() {
    let value = "100";
    let value: i32 = value.parse().unwrap();
    let value = value * 2;

    println!("Final value: {}", value);
}

Output: Final value: 200

WHY THIS WORKS AS AN ANSWER
------------------------------
Each of the three let value lines creates a genuinely NEW binding that
shadows the previous one — the first holds a string ("100"), the
second shadows it with the parsed i32 (100), and the third shadows
THAT with the doubled result (200). Each version is a completely
separate variable that happens to share the name "value"; nothing
about the original string variable was ever mutated.

WHY THIS REQUIRED let EACH TIME, NOT PLAIN REASSIGNMENT: plain
reassignment (value = ...) would require value to have been declared
mut, and critically, mutation can NEVER change a variable's type — a
mut string variable can only ever be reassigned to another string, not
suddenly become an i32. Shadowing is the only mechanism in Rust that
allows the type itself to change across "reassignments" (as this
chapter's own string-to-i32 example demonstrated), which is exactly
why the string-to-number-to-doubled-number progression here needed let
at every step: each step is really a distinct variable of a distinct
type, not the same variable being mutated in place.