test: migrate math/base/special/atanf to ULP-based testing

[Planeshifter]

Resolves a part of #11352.

Description

What is the purpose of this pull request?

This pull request:

• migrates test/test.js and test/test.native.js in math/base/special/atanf from relative-tolerance (EPS-based) assertions to ULP-difference assertions via @stdlib/number/float32/base/assert/is-almost-same-value (the single-precision variant, following the precedent of math/base/special/acothf and math/base/special/atanhf)
• uses the minimum required ULP values per test block, verified by direct ULP-difference computation over the test fixtures via @stdlib/number/float32/base/ulp-difference: the tiny_negative, tiny_positive, huge_negative, and huge_positive blocks match exactly (ULP 0) and therefore use plain t.strictEqual, the [ -0.8, 0.8 ] block requires 3 ULPs (the original test used a widened 1.8 * EPS tolerance there), and all remaining blocks require 1 ULP
• applies the same ULP values to the native test file; the native (C) results do not diverge from the JavaScript results, so no compiler-optimization tolerance note is needed

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: Migrate math/base/special packages from relative tolerance testing to ULP difference testing (tracking issue) #11352

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.

Minimality of the ULP values was verified by recomputing the maximum ULP difference per fixture block and by confirming that reducing the [ -0.8, 0.8 ] block from 3 to 2 ULPs fails in both the JavaScript and native test suites. The full JavaScript and native test suites pass with these values (6514/6514 assertions each).

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

This PR was written primarily by Claude Code, including the test migration, minimum-ULP discovery, and an independent adversarial verification pass that recomputed the per-block ULP values from the fixtures.

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@stdlib-js/reviewers

[stdlib-bot]

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/atanf	$\\color{green}284/284$ $\\color{green}+100.00\\%$	$\\color{green}21/21$ $\\color{green}+100.00\\%$	$\\color{green}3/3$ $\\color{green}+100.00\\%$	$\\color{green}284/284$ $\\color{green}+100.00\\%$

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