feat: implement mean and variance for half-normal distribution

[kshitijgarg2811-oss]

Addresses #9416

Description

What is the purpose of this pull request?

This pull request:

• Implements the mean and variance sub-packages for the Half-Normal distribution.
• Adds unit tests and documentation for both properties.
• Establishes the initial directory structure for @stdlib/stats/base/dists/halfnormal.

This is the first step in addressing the tracking issue #9416. Future PRs will add the PDF, CDF, and other distribution properties.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: Tracking issue for complete implementation of @stdlib/stats/base/dists/halfnormal package #9416

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

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• Test/benchmark generation
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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/halfnormal/mean	$\color{green}70/70$ $\color{green}+0.00%$	$\color{green}5/5$ $\color{green}+0.00%$	$\color{green}1/1$ $\color{green}+0.00%$	$\color{green}70/70$ $\color{green}+0.00%$
stats/base/dists/halfnormal/variance	$\color{green}71/71$ $\color{green}+0.00%$	$\color{green}6/6$ $\color{green}+0.00%$	$\color{green}1/1$ $\color{green}+0.00%$	$\color{green}71/71$ $\color{green}+0.00%$

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