feat : add C implementation for stats/base/ndarray/dnanmeanpw

[ujjwalv01]

Resolves a part of #14034

Description

What is the purpose of this pull request?

This pull request:

• Adds a C implementation for @stdlib/stats/base/ndarray/dnanmeanpw (computes the arithmetic mean of a one-dimensional double-precision floating-point ndarray, ignoring NaN values and using pairwise summation).
• Updates the package to dynamically load the native C implementation when available.
• Adds C benchmarks, C examples, and updates the test suite to cover the new native C addon.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC] : add C implementations for @stdlib/stats/base/ndarray packages (tracking issue) #14034

Questions

Any questions for reviewers of this pull request?

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

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
• Documentation (including examples)
• Research and understanding

Disclosure

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N/A

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/ndarray/dnanmeanpw	$\\color{green}176/176$ $\\color{green}+100.00\\%$	$\\color{green}7/7$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}176/176$ $\\color{green}+100.00\\%$

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