feat: add C implementation for blas/ext/base/ndarray/dcusumkbn2

[AtharvaPatange]

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/dcusumkbn2.
• 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?

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

AI assistance was used for reviewing changes and researching repository conventions.

I've checked each file and verified the result locally: the addon compiles without warnings, both the JavaScript and native test suites pass with the native results matching the JavaScript implementation, the C example and benchmarks run correctly, and ESLint reports no errors. Happy to make any adjustments.

---

@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
blas/ext/base/ndarray/dcusumkbn2	$\\color{green}214/214$ $\\color{green}+100.00\\%$	$\\color{green}7/7$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}214/214$ $\\color{green}+100.00\\%$

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