fix: respect start/end boundaries in intro_sort heap-sort fallback

[dajiaohuang]

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• Fix a bug or typo in an existing algorithm?
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The heap_sort() call in intro_sort() ignored the start and end parameters, causing elements outside the requested range to be reordered when max_depth reached zero and the algorithm fell back to heap sort.

Changes:

• Add optional start/end parameters to heapify() and heap_sort() helpers within intro_sort.py, with default values preserving existing whole-array behavior (backward compatible)
• Update the intro_sort() call site at line 194 to pass the correct subrange (start, end) to heap_sort()

Validation:

• All 41 existing doctests pass
• Manually verified the reproduction case from issue intro_sort heap-sort fallback ignores start and end boundaries #15434 produces correct output
• ruff check sorts/intro_sort.py passes

Fixes #15434