fix: type alternative as string union in ztest results struct-factory

[Planeshifter]

Description

What is the purpose of this pull request?

This pull request types the alternative field as the string-union Alternative (two-sided, greater, less) instead of number, matching the sibling float64/float32 results declarations. Adds the Alternative type alias used by both the optional Results member and the required Struct field.

This is part of a series of follow-up PRs addressing findings from a TypeScript declaration audit of the stats namespace (documentation-only fixes landed separately in #12482).

Related Issues

Does this pull request have any related issues?

No.

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

This change was surfaced by a Claude Code audit of the stats namespace TypeScript declarations. The fix was verified against the implementation and declaration test before submission, and reviewed by myself.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/ztest/one-sample/results/struct-factory	$\color{green}142/142$ $\color{green}+100.00\%$	$\color{green}3/3$ $\color{green}+100.00\%$	$\color{green}1/1$ $\color{green}+100.00\%$	$\color{green}142/142$ $\color{green}+100.00\%$

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

[Planeshifter]

Closing as a false positive. The audit flagged alternative: number as inconsistent with the float64/float32 results objects (which expose a string union), but this struct stores alternative as an int8 field (castingMode: 'none') holding the resolved enum code — per the README, values are assigned via resolveEnum( 'two-sided' ), i.e. a number. The original alternative: number typing is correct, so no change is warranted.