Fix introsort heap fallback ignoring range boundaries

[Miladkhoshdel]

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• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
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Fixes #15434

When intro_sort() reaches its depth limit, its heap-sort fallback sorts the entire list instead of the requested [start:end] range. This moves elements outside the range.

Apply heap sort only to the selected slice and write it back into the original list. Add two pytest regression cases covering immediate fallback and fallback after partitioning. Existing doctests remain unchanged.

For example, sorting indices 1 through 4 of [100, 4, 3, 2, 1, -100] should produce [100, 1, 2, 3, 4, -100]. Previously, the fallback produced [-100, 1, 2, 3, 4, 100].

Validation:

• Both regression cases failed before the fix and passed afterward.
• Sorting tests and introsort doctests: 368 passed.
• Direct introsort doctest run: 41 passed.
• Repository-wide pre-commit checks passed.

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