test: migrate math/base/special/atan2d to ULP-based testing

[ujjwalv01]

Resolves a part of #11352.

Description

What is the purpose of this pull request?

This pull request:

• migrates the tests for math/base/special/atan2d from computed relative tolerance (EPS-scaled delta <= tol) checks to ULP-based assertions;
• replaces the @stdlib/math/base/special/abs require with @stdlib/number/float64/base/assert/is-almost-same-value. (The EPS constant is removed as it is no longer required).
• both test/test.js and test/test.native.js are modified to use the new ULP assertions.

ULP bounds (one per fixture set, each tightened to the measured minimum):

fixture set	previous tolerance	ULP bound
positive_positive	1.2 * EPS	2
negative_positive	2.3 * EPS	4
negative_negative	2.0 * EPS	2

Each bound is the smallest non-negative integer for which every fixture in the corresponding set passes. Starting from a high bound and lowering, the suite passes at the values above, and decrementing each bound by one produces exactly one failure per set, confirming that no bound can be tightened further.

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.

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

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• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

Disclosure

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{{TODO: add disclosure if applicable}}

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Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/atan2d	$\\color{green}192/192$ $\\color{green}+100.00\\%$	$\\color{green}5/5$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}192/192$ $\\color{green}+100.00\\%$

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