feat: implemented max-nth-double-factorial missing constant

[navyansh007]

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Resolves #9061.

Description

What is the purpose of this pull request?

This pull request:

• Adds @stdlib/constants/float16/max-nth-double-factorial constant
• Implements the maximum nth value for which the double factorial can be stored in half-precision floating-point format without overflow
• The constant value is 12, since 12!! = 46080 fits within float16 max (65504), while 13!! = 135135 overflows

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: tracking issue for missing float16 constants #9061 (tracking issue for missing float16 constants)

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.

Implementation Details:

• Constant value: 12
• Calculation: 12!! = 12 × 10 × 8 × 6 × 4 × 2 = 46,080 (within float16 range)
• Overflow check: 13!! = 135,135 (exceeds float16 max of 65,504)
• Follows the same structure as existing float32/float64 max-nth-double-factorial constants
• Includes full test coverage, TypeScript declarations, C header, and documentation

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

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself."

This PR was partially written with the help of Claude Code for initial implementation, following the existing patterns from float32/float64 max-nth-double-factorial constants and merged float16 PRs. The constant value was calculated programmatically, and the entire implementation was thoroughly reviewed and verified by me to ensure correctness and adherence to project standards.

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Coverage Report

Package	Statements	Branches	Functions	Lines
constants/float16/max-nth-double-factorial	$\color{green}51/51$ $\color{green}+100.00%$	$\color{green}1/1$ $\color{green}+100.00%$	$\color{green}0/0$ $\color{green}+100.00%$	$\color{green}51/51$ $\color{green}+100.00%$

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