lru_cache call count

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Without caching, `fib(4)` would make 9 calls because the naive recursion recomputes overlapping subproblems. With `lru_cache`, each unique argument is computed exactly once. The unique calls are `fib(4)`, `fib(3)`, `fib(2)`, `fib(1)`, and `fib(0)` - that's 5 distinct values. After `fib(2)` is cached, `fib(3)`'s second recursive call hits the cache instead of incrementing `calls`. This is exactly the exponential-to-linear speedup that makes `lru_cache` so valuable for top-down dynamic programming.