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

[dajiaohuang]

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(), with default values preserving existing whole-array behavior
• Update the intro_sort() call site to pass the correct subrange to heap_sort()
• Add a regression doctest for the subrange heap-sort fallback case

Validation:

• All 43 existing doctests pass
• New regression doctest for issue intro_sort heap-sort fallback ignores start and end boundaries #15434 passes
• ruff check passes

Fixes #15434

[algorithms-keeper]

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