feat: add blas/ext/base/dcartesian-product

[headlessNode]

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Resolves stdlib-js/metr-issue-tracker#173.

Description

What is the purpose of this pull request?

This pull request:

• add blas/ext/base/dcartesian-product

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• Resolves [RFC]: add blas/ext/base/dcartesian-product metr-issue-tracker#173

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No.

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No.

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• Read, understood, and followed the contributing guidelines.

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C implementation and relevant docs, files were done by Claude Code.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
blas/ext/base/dcartesian-product	$\color{green}496/496$ $\color{green}+0.00%$	$\color{green}38/38$ $\color{green}+0.00%$	$\color{green}4/4$ $\color{green}+0.00%$	$\color{green}496/496$ $\color{green}+0.00%$

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

[kgryte]

Left initial comments. This PR needs quite a bit of clean-up.

[kgryte]

@headlessNode Why did you decide to deviate from the convention in https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/blas/ext/base/dcartesian-square#notes where pairs are always stored as rows?

We should be consistent. The storage layout is not the same as the logical layout.

[kgryte]

Suggested change

	* @param {NonNegativeInteger} M - number of indexed elements of `x`
	* @param {NonNegativeInteger} N - number of indexed elements of `y`
	* @param {NonNegativeInteger} M - number of indexed elements in `x`
	* @param {NonNegativeInteger} N - number of indexed elements in `y`

Applies here and elsewhere.

[kgryte]

I am not convinced that these notes actually reflect the implementation. The implementation seems to be such that pairs are always stored as rows, which is as expected.

So it seems that these notes are what needs to be fixed here and throughout this PR.

[kgryte]

Other than all the incorrect notes, this PR is coming along.