feat: add C implementation for invgamma/quantile

[ujjwalv01]

Resolves #3677.

Description

What is the purpose of this pull request?

This pull request:

• Adds a native C implementation for the @stdlib/stats/base/dists/invgamma/quantile package.
• Adds N-API bindings to wrap the C logic for Node.js usage.
• Updates lib/index.js to intelligently fall back to the pure JavaScript implementation if the native addon fails to compile or load.
• Adds C and JS native benchmarks, C examples, and native tests.
• Updates the package documentation (README.md) to document the newly exposed C API.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: Add C implementations to inverse gamma distribution functions (tracking issue) #3669

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 consulted an AI assistant to understand the codebase structure and how native C addons and bindings are scaffolded in stdlib, but the proposed changes were manually implemented and integrated.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/invgamma/quantile	$\\color{red}298/314$ $\\color{green}+0.00\\%$	$\\color{red}15/16$ $\\color{green}+0.00\\%$	$\\color{red}3/4$ $\\color{green}+0.00\\%$	$\\color{red}298/314$ $\\color{green}+0.00\\%$

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