test: migrate stats/base/dists/signrank/pdf 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/signrank/pdf from relative tolerance assertions to ULP-based assertions, replacing the delta/tol computation (delta = abs( y - expected[ i ] ), tol = 20.0 * EPS * abs( expected[ i ] )) with isAlmostSameValue( y, expected[ i ], 5 ) via @stdlib/assert/is-almost-same-value.
• drops the now unused @stdlib/math/base/special/abs and @stdlib/constants/float64/eps requires, along with the delta and tol variable declarations.
• updates the affected assertion messages to 'returns expected value'.

Files changed (test files only):

• test/test.pdf.js
• test/test.factory.js
• test/test.native.js

Final ULP constant: 5 (used in all three test files).

This was tightened from an initial bound of 64. The maximum measured ULP difference over the 100-element fixture set is exactly 5, and is identical for the JavaScript and C implementations:

test.pdf.js      (JS)       -> max ulpDifference = 5
test.factory.js  (JS)       -> max ulpDifference = 5
test.native.js   (native C) -> max ulpDifference = 5

worst case: x = 51, n = 23
  actual   = 0.0003389120101928709
  expected = 0.00033891201019287115

The JavaScript and C implementations returned bit-identical results for all 100 fixture cases. A bound of 4 ULP fails in all three files (one failing assertion each), confirming 5 is the minimum.

Note that the previous bound in test.native.js was looser than the JavaScript files (80.0 * EPS vs. 20.0 * EPS); since the measured ULP difference is the same for both implementations, all three files now use the same bound.

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.

• The test suite was run twice at the final bound of 5 ULP to confirm determinism; the native add-on was built locally (node-gyp rebuild) so that the test.native.js assertions were actually exercised rather than skipped. All four test files pass (342 assertions, 0 failures) on both runs.
• Linting is clean for the changed files using the project's test ESLint configuration (make eslint-tests TESTS_FILTER=".*/stats/base/dists/signrank/pdf/.*").
• The editorconfig-checker step could not run in this environment because its binary is downloaded from a GitHub release that outbound network policy blocked. EditorConfig compliance was instead verified manually for the changed files: LF line endings, UTF-8 (ASCII), tab indentation, no trailing whitespace, and a final newline.

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, following the idiom established by previously merged conversions in the same family (e.g. stats/base/dists/hypergeometric/cdf, stats/base/dists/beta/logpdf). The ULP bound was measured empirically rather than guessed.

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@stdlib-js/reviewers

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Generated by Claude Code

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/signrank/pdf	$\\color{red}359/361$ $\\color{green}+99.45\\%$	$\\color{red}36/37$ $\\color{green}+97.30\\%$	$\\color{green}5/5$ $\\color{green}+100.00\\%$	$\\color{red}359/361$ $\\color{green}+99.45\\%$

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