Add factorial_iterative implementation in Python

maths/factorial_iterative.py

@@ -0,0 +1,16 @@
+def factorial_iterative(n: int) -> int:
+    """
+    Return the factorial of n using an iterative approach.
+    Raises ValueError for negative inputs.
+    """
+    if n < 0:
+        raise ValueError("Input must be a non-negative integer")
+    result = 1
+    for i in range(1, n + 1):
+        result *= i
+    return result
+
+
+if __name__ == "__main__":
+    # simple demonstration
+    print(factorial_iterative(5))  # expected 120