usePrevious Hook

Implement usePrevious(initialValue) — a hook simulation that:

• Returns a get() function to read the previous value.
• Returns an update(newValue) function that shifts current → previous.

On the first call, get() returns null.

solve(initial, updates) creates the hook, applies updates, and returns
the recorded history of [previous, current] pairs.

const [get, update] = usePrevious(0);
// get() → null
update(1); // get() → 0
update(2); // get() → 1