📝 **Task:** Write `max_sum_window(arr, k)` returning the maximum sum of any contiguous subarray of length `k`. Print `max_sum_window([2, 1, 5, 1, 3, 2], 3)`. 📋 Implement the function above. Test cases run automatically. 💡 **Hint:** Re-read the theory if you get stuck — the pattern is right there.