feat: add LeetCode problem 92

[darshan0548]

Adds an iterative C solution for LeetCode #92 "Reverse Linked List II" —
reverses the sublist of a singly linked list between positions left and
right (1-indexed), using a dummy node and the standard prev/curr
pointer-reversal technique.

Description of Change

Iterative reversal of a sublist (positions left to right) in a singly
linked list, using a dummy node to simplify the edge case where left == 1.

References

N/A

Checklist

• Added description of change
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• PR title follows semantic commit guidelines
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Notes: Iterative solution, O(n) time, O(1) space.