test: migrate stats/base/dists/logistic/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/logistic/pdf from relative tolerance testing (delta/tol computed from EPS) to ULP difference testing using @stdlib/assert/is-almost-same-value.
• updates test/test.pdf.js, test/test.factory.js, and test/test.native.js. No other files are changed, and no source, benchmark, example, or documentation files are touched.

The ULP bound used in every migrated assertion is 4, and this is the measured minimum. Across all three Julia fixture sets (positive_mean, negative_mean, large_variance; 2980 compared values after applying the existing null/underflow guards), the maximum observed ULP difference between the returned value and the expected value is exactly 4, for both the main export and the function returned by factory. Lowering the bound to 3 produces 14 failures; 4 is therefore the tightest integer bound that passes over the full fixture set.

The previous per-loop tolerances were 3.0 * EPS for the positive-/negative-mean fixtures and 3.0 * EPS for the large-variance fixture, so the new bound is a comparable, and in ULP terms tighter and better-defined, accuracy budget.

The full package test suite (6002 assertions) passes, and was run twice at the final bound with identical results, so the bound is not sensitive to run-to-run variation.

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?

• test/test.native.js is skipped locally because the native add-on was not built in this environment. To avoid guessing at the bound for the C implementation, src/main.c was compiled directly against its declared dependency closure and evaluated over the same fixtures: the maximum ULP difference is also 4 at -O0, -O2, -O3, and -O3 -ffp-contract=fast -march=native (i.e., with FMA contraction enabled), so the same bound of 4 is used in test/test.native.js. Reviewers may wish to confirm this against a real add-on build.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

• The existing guard conditions (expected[i] !== null and !(expected[i] === 0.0 && y < EPS)) are preserved unchanged, so the set of compared values is exactly the same as before. The EPS import is retained for those guards; the now-unused abs import is replaced by the isAlmostSameValue import.
• The change mirrors the idiom settled on in prior conversions in this family, e.g. stats/base/dists/levy/pdf (test: migrate stats/base/dists/levy/pdf to ULP-based assertions #14985).

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 by Claude Code, running unattended as a scheduled task. The ULP bound was determined empirically by measuring the maximum ULP difference over the full fixture set rather than by guessing, and was verified by confirming that the next lower bound fails.

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https://claude.ai/code/session_01N5UvaSs85zP3Le1hoAaHvK

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/logistic/pdf	$\\color{green}314/314$ $\\color{green}+0.00\\%$	$\\color{green}29/29$ $\\color{green}+0.00\\%$	$\\color{green}4/4$ $\\color{green}+0.00\\%$	$\\color{green}314/314$ $\\color{green}+0.00\\%$

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