test: migrate stats/base/dists/f/pdf to ULP-based assertions

[aryan7071]

Progresses #11352

Description

What is the purpose of this pull request?

This pull request:

• migrates the test suite for stats/base/dists/f/pdf from relative tolerance (abs/EPS-based) assertions to ULP difference assertions using @stdlib/assert/is-almost-same-value.
• updates test/test.factory.js, test/test.pdf.js, and test/test.native.js. test/test.js contains no fixture-based numeric assertions and required no changes.
• removes the now-unused abs and EPS imports and the corresponding delta/tol variable declarations, collapsing each if/else exact-match-or-tolerance branch into a single isAlmostSameValue assertion.

Each of the six fixture blocks (bothLarge, largeD1, largeD2 tested against both Julia and Boost C++ reference values) required its own independently-determined minimum ULP bound. The pure JS implementation (test.factory.js, test.pdf.js) and the native binding (test.native.js) required different bounds, since the native binding diverges from the JS implementation at the level of individual floating-point bits, similar to compiler-optimization-driven divergence noted in other previously migrated packages.

Measured minimum ULP bounds:

Fixture	Reference	JS ULP	Native ULP
both_large	Julia	4003	230
large_d1	Julia	95	137
large_d2	Julia	103	122
both_large	Boost C++	148	1474
large_d1	Boost C++	105	694
large_d2	Boost C++	1285	660

Each bound is the exact maximum observed ULP difference over its fixture, measured separately against the pure JS implementation and a locally-built native addon.

Two of the JS bounds are notably larger than the rest: both_large/Julia (4003) and large_d2/Boost C++ (1285). Both outlier cases occur at inputs where d1 is very close to 2.0 — a value this distribution already treats as a special boundary elsewhere in the test suite (see the existing d1<2.0/d1=2.0/d1>2.0 boundary tests). Inspecting these cases directly shows actual and expected values still agree closely in absolute/relative terms; the large ULP counts reflect the fine representable spacing of floating-point values in this region rather than a correctness issue.

The pre-existing "baseline case" test in test.native.js (a single hardcoded input/output check, unrelated to the six fixture-driven blocks above) already used isAlmostSameValue with ULP=2 prior to this PR and was not modified.

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

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{{TODO: add disclosure if applicable}}

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/f/pdf	$\\color{red}599/647$ $\\color{green}+92.58\\%$	$\\color{red}62/66$ $\\color{green}+93.94\\%$	$\\color{green}6/6$ $\\color{green}+100.00\\%$	$\\color{red}599/647$ $\\color{green}+92.58\\%$

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