test: migrate stats/base/dists/normal/cdf to ULP-based assertions

[kgryte]

Resolves a part of #11352.

Description

What is the purpose of this pull request?

This pull request:

• migrates the tests for stats/base/dists/normal/cdf from relative tolerance testing to ULP difference testing, as described in [RFC]: Migrate math/base/special packages from relative tolerance testing to ULP difference testing (tracking issue) #11352.

Changes are confined to test/test.cdf.js, test/test.factory.js, and test/test.native.js. In each fixture loop, var delta/var tol and the abs( y - expected[i] ) <= EPS * abs( expected[i] ) comparison are replaced with t.strictEqual( isAlmostSameValue( y, expected[ i ], N ), true, 'returns expected value' ), and the now-unused @stdlib/math/base/special/abs and @stdlib/constants/float64/eps requires are removed in favor of @stdlib/assert/is-almost-same-value.

The ULP bounds were measured against the full fixture set (1000 cases per fixture) and tightened to the minimum passing integer:

Fixture	Previous tolerance	ULP bound
positive_mean.json	1600.0 * EPS	2207
negative_mean.json	1600.0 * EPS	2433
large_variance.json	250.0 * EPS	4

Each bound is the measured maximum ULP difference over its fixture, and is minimal: lowering any of them by one causes that fixture to fail. The same bounds apply to all three test files, as the C implementation returns bit-identical values to the JavaScript implementation for all 3000 fixture cases (verified locally with the native add-on built), so no JS vs. C divergence needed to be accounted for.

The full package test suite (make test TESTS_FILTER=".*/stats/base/dists/normal/cdf/.*") passes, including test.native.js against a locally built add-on, and was run twice at the final ULP values to confirm the results are deterministic. make eslint-tests is clean for the package.

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

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 authored by Claude Code, following the migration pattern established in previously merged conversions (e.g. #14267, #14258, #14272). The ULP bounds were determined empirically by measuring ULP differences across the full fixture set and verifying minimality, rather than guessed.

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/normal/cdf	$\\color{green}299/299$ $\\color{green}+100.00\\%$	$\\color{green}25/25$ $\\color{green}+100.00\\%$	$\\color{green}4/4$ $\\color{green}+100.00\\%$	$\\color{green}299/299$ $\\color{green}+100.00\\%$

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