feat: add math/base/special/cexpf

[nirmaljb]

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Progresses: #649

Description

What is the purpose of this pull request?

This pull request:

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

Related Issues

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This pull request has the following related issues:

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

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I consulted Claude Opus 4.6 (Thinking) to find reference packages, but the proposed changes were fully authored manually by myself.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/cexpf	$\\color{green}200/200$ $\\color{green}+0.00\\%$	$\\color{green}16/16$ $\\color{green}+0.00\\%$	$\\color{green}2/2$ $\\color{green}+0.00\\%$	$\\color{green}200/200$ $\\color{green}+0.00\\%$

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

[stdlib-bot]

⚠️ Tracking Issue Closure Warning ⚠️

I noticed your PR description contains closing keywords ("Resolves", "Closes", or "Fixes") referencing a "Tracking Issue".

Why this matters:
Tracking issues should typically remain open until all related sub-issues are completed. GitHub automatically closes issues with such closing keywords when the PR is merged. For more information, see GitHub's documentation on using keywords in issues and pull requests.

Required action:
Use "Progresses" instead to reference the tracking issue without automatically closing it.

Thank you for your contribution to the project!

[anandkaranubc]

Blocked because expf isn't implemented yet! 😞

[kgryte]

@anandkaranubc Looks like we have had two PRs attempt to implement. Feel free to take either one over. One seems to be a recent submission so YMMV.

[anandkaranubc]

Looks good, after the changes :)

[kgryte]

Other than the test descriptions, this is looking good.

@anandkaranubc We need to add follow-up issues to add assign and strided methods to both cexpf and cexp, similar to what we do in complex/float64/base/add. These could be decent METR tasks.

[kgryte]

LGTM