Add heap sort algorithm

Implement heap sort algorithm for the python-sort-algorithm project.

Heap sort is an efficient comparison-based sorting algorithm with O(n log n) time complexity in all cases (best, average, and worst).

Tasks:

• Implement heap_sort() function in sort.py
• Add test cases for heap_sort
• Update README.md with algorithm documentation
• Update main.py demo

Acceptance Criteria:

• All existing tests pass
• New heap_sort tests pass
• Code follows existing project style