[AlexXuz]

This PR preserves input order for equal-key records returned by nlargest and nsmallest.

• Use decreasing encounter indices in both heaps, retaining earlier items at the cutoff and returning ties in arrival order.
• Preserve key equality, including the same-object identity shortcut, through ReverseLT so tuple comparisons can use their encounter index.
• Add 62 parametrized cases covering both functions, async keys, dict payloads, heap replacement, limits larger than the input, and a shared non-reflexive key.

Closes #241

Validation on Linux:

• Original base: 44 new cases fail, 18 pass.
• CPython 3.12.14: 464 tests pass.
• CPython 3.13.5: 460 pass, 4 existing version-specific descriptor skips.
• Black, flake8, mypy and pyright pass.

Other Python versions and operating systems were not run locally. The change does not modify merge or the active work in #238. AI-assisted contribution with independent correctness review.

[maxfischer2781]

Hi @AlexXuz, thanks for the bug report and solution draft. Please be mindful that AI-assisted contributions don't have the best track record here and that I am aware your attention is spread over dozens of PRs.

I have given your PR a first run of inspections. Please mark it as ready for review once appropriate.