Heap Sort with Your Own Sift-Down

Implement heap sort in place without heapq:

• sift_down(a, i, n): restore the max-heap property for the subtree at i within a[:n]
• heapify(a): build a max-heap bottom-up in O(n)
• heap_sort(a): repeatedly swap the max to the end and shrink the heap; sorts a ascending in place and returns None