feat: add C implementation for blas/ext/base/ndarray/sxmy

[Devansh-18155]

Part of #14819.

Description

What is the purpose of this pull request?

This pull request:

• adds a C implementation for @stdlib/blas/ext/base/ndarray/sxmy.
• adds a Node-API addon and tryRequire dispatcher with a fallback to the JavaScript implementation when the native addon is unavailable.
• adds JavaScript and C benchmarks, along with a C example.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: add C implementations for blas/ext/base/ndarray packages #14819

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

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

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself.".

I used AI for understanding the benchmarks, and documentation generation of the following stdlib patterns.

---

@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
blas/ext/base/ndarray/sxmy	$\\color{green}188/188$ $\\color{green}+100.00\\%$	$\\color{green}7/7$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}188/188$ $\\color{green}+100.00\\%$

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

[ujjwalv01]

@Devansh-18155 , Is this a duplicate of #14879?