feat: add ndarray/base/nans

[headlessNode]

Resolves stdlib-js/metr-issue-tracker#210.

Description

What is the purpose of this pull request?

This pull request:

• add ndarray/base/nans

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• Resolves [RFC]: add ndarray/base/nans metr-issue-tracker#210

Questions

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No.

Other

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No.

Checklist

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

• 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?

• Yes
• No

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• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
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Disclosure

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Primarily written by Claude Code.

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@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
ndarray/base/nans	$\color{green}103/103$ $\color{green}+100.00%$	$\color{green}5/5$ $\color{green}+100.00%$	$\color{green}1/1$ $\color{green}+100.00%$	$\color{green}103/103$ $\color{green}+100.00%$

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

[kgryte]

The main thing is adding (confirming) support for "generic" dtypes.

[kgryte]

LGTM after clean-up.

[kgryte]

For METR,

review_time: 15 mins