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

[Planeshifter]

Resolves a part of #11352.

Description

This pull request migrates the tests for @stdlib/math/base/special/erfcinv (test/test.js and test/test.native.js) from relative-tolerance (EPS-scaled delta/tol) comparisons to ULP-difference assertions using @stdlib/assert/is-almost-same-value.

The seven fixture blocks each had a distinct tolerance multiplier in the original tests (3.0, 3.0, 3.0, 13.0, 14.0, 9.0, and 1.0 times EPS). Final ULP constants were determined empirically by computing the actual maximum ULP distance between the JS implementation's output and the Julia fixture data for each block, then confirming that the tests fail at N - 1 and pass deterministically (verified across two consecutive runs) at the chosen N:

Fixture interval	Final ULP constant
[0.5, 1.5]	5
[0.25, 0.5]	3
[1.5, 1.75]	4
[1.75, 1.9998]	20
[0.0002, 0.25]	19
[1.9998, 1.9999..8]	11
[1.9999..8, 2]	1

The native add-on could not be built in this environment, so test.native.js assertions are skipped, but the file was updated identically to test.js (native add-on is expected to produce the same output as the JS implementation).

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

No.

Other

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Checklist

• Read, understood, and followed the contributing guidelines.

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Generated by Claude Code

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/erfcinv	$\\color{red}630/659$ $\\color{green}+95.60\\%$	$\\color{red}43/56$ $\\color{green}+76.79\\%$	$\\color{green}7/7$ $\\color{green}+100.00\\%$	$\\color{red}630/659$ $\\color{green}+95.60\\%$

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