feat: add incrnanmmeanvar (moving mean/variance with NaN skipping) (fixes #5592)

[anoofmhd]

Resolves #5592.

Description

What is the purpose of this pull request?

This pull request:

• Adds a new incremental moving mean and variance accumulator: @stdlib/stats/incr/nanmmeanvar.

• This module behaves similarly to @stdlib/stats/incr/mmeanvar, but skips NaN values instead of propagating them.

• Includes implementation, tests, documentation, TypeScript definitions, REPL help file, benchmarks, and examples.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• #5592

Questions

Any questions for reviewers of this pull request?

No.

Other

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No additional information.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• [x] Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• [x] Yes
• [ ] No

If you answered "yes" above, how did you use AI assistance?

• [ ] Code generation (e.g., when writing an implementation or fixing a bug)
• [ ] Test/benchmark generation
• [x] Documentation (including examples)
• [x] Research and understanding

Disclosure

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I used ChatGPT to assist in understanding spec expectations and drafting documentation wording. The implementation and test logic were written and verified manually.

---

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/incr/nanmmeanvar	$\color{green}140/140$ $color{green}+100.00%$	$\color{green}11/11$ $color{green}+100.00%$	$\color{green}2/2$ $color{green}+100.00%$	$\color{green}140/140$ $color{green}+100.00%$

The above coverage report was generated for the changes in this PR.

[kgryte]

/stdlib update-copyright-years

[anoofmhd]

@Planeshifter @stdlib-bot @kgryte @rotu can you please review it and tell me if any changes needed or the PR is ready to merge.