feat: add math/base/special/expitf

[Devansh-18155]

Resolves part of #649.

Description

What is the purpose of this pull request?

This pull request:

• Adds the @stdlib/math/base/special/expitf package for evaluating the standard logistic function (sigmoid) in single-precision floating-point format: 1 / (1 + exp(-x)).
• Provides a JavaScript implementation, pure C implementation (stdlib_base_expitf), and Node-API native add-on bindings.
• Includes TAP-compliant unit tests, test fixtures generated using SciPy, benchmarks, TypeScript definitions, examples (JS and C), REPL documentation, and package documentation.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: Add C implementations to base special math functions (tracking issue) #649

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.".

This PR was developed with assistance of Claude Code to implement the single-precision JavaScript, C, and documentation files according to stdlib conventions.

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Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/expitf	$\\color{green}191/191$ $\\color{green}+100.00\\%$	$\\color{green}7/7$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}191/191$ $\\color{green}+100.00\\%$

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