Loop Detection

Given a circular linked list, implement an algorithm that returns the node at the beginning of the loop.

Definition: A circular linked list is one where a node's next pointer points to an earlier node, creating a loop.

Example:

A → B → C → D → E → C  (C is the start of the loop)
Output: C

For this problem, represent the list as a Python list and an integer loop_start index. Return the index of the loop start.