Add heap sort algorithm

Description

Add heap sort implementation to the sorting algorithms collection.

Requirements

• Implement heap_sort() function with O(n log n) time complexity
• Add heapify helper function for max-heap maintenance
• Update tests to include heap sort validation
• Add heap sort to demo program
• Update README with heap sort complexity information

Algorithm Details

• Time Complexity: O(n log n) best/worst case
• Space Complexity: O(1) (in-place sorting)
• Stable: No
• Method: Builds max-heap and repeatedly extracts maximum element