feat: add Wald distribution variance package

[manit2004]

Progresses #209 .

Description

What is the purpose of this pull request?

• This is a WIP pull request. Currently the core functionalities of Wald Distribution variance package has been implemented. Feedback needed on this implementation.

Tests, docs, examples and benchmark is yet to added, pending approval of the core implementation.

Related Issues

Does this pull request have any related issues?

• None

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.".

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@stdlib-js/reviewers

[Planeshifter]

This line is missing an indentation level - it should have two tabs to match the other conditions above it.

Suggested change

	mu <= 0.0
	mu <= 0.0

[Planeshifter]

Thanks!

First thing I noticed is that README.md is missing and should be added.