fix: preserve sign when rounding the smallest subnormal

[manit2004]

Resolves #15666

Description

What is the purpose of this pull request?

This pull request:

• Fixes round2(-Number.MIN_VALUE) returning the positive smallest subnormal instead of the negative one. The function normalizes x to its absolute value up front and tracks the original sign separately (sign), then multiplies back by sign on every return path except one; the early-return for "this is exactly the smallest subnormal, can't round it any further" returned the bare (already-absolute-valued) x instead of sign * x. Adds a test covering the negative case, since the existing suite only checked the positive smallest subnormal (where sign is +1, so the bug was invisible).

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• None

Questions

Any questions for reviewers of this pull request?

No.

Other

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Repro:

var round2 = require( '@stdlib/math/base/special/round2' );

var v = round2( -Number.MIN_VALUE );
// returns 5e-324, should return -5e-324

Verified before/after: 39/40 passing (1 new failing test) on the unfixed code, 40/40 after the fix.

Checklist

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• Read, understood, and followed the contributing guidelines.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/round2	$\\color{green}218/218$ $\\color{green}+100.00\\%$	$\\color{green}19/19$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}218/218$ $\\color{green}+100.00\\%$

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

[kgryte]

LGTM. Thanks for fixing, @manit2004!