feat: add C implementation for @stdlib/stats/base/dists/chi/kurtosis

[rautelaKamal]

Description

This pull request adds a native C implementation for @stdlib/stats/base/dists/chi/kurtosis, resolving issue #3491.

The excess kurtosis of a chi distribution with degrees of freedom k is:

kurtosis = (2 / sigma2) * (1 - mu * sigma * g1 - sigma2)

where mu = mean(k), sigma2 = variance(k), sigma = sqrt(sigma2), and g1 = skewness(k).

Changes

• Adds src/main.c implementing the kurtosis formula in C
• Adds src/addon.c N-API wrapper using STDLIB_MATH_BASE_NAPI_MODULE_D_D
• Adds include/stdlib/stats/base/dists/chi/kurtosis.h header
• Adds build files (binding.gyp, include.gypi, src/Makefile)
• Adds manifest.json with required C library dependencies (mean, variance, skewness, sqrt, is-nan)
• Updates lib/index.js to load native addon with pure-JS fallback
• Adds lib/native.js native addon loader
• Adds test/test.native.js for native addon testing
• Adds test/test.main.js for JS fallback coverage (100% coverage)
• Adds native benchmarks and C examples

Related Issues

Ref: #3491

Checklist

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/chi/kurtosis	$\color{green}180/180$ $\color{green}+100.00%$	$\color{green}10/10$ $\color{green}+100.00%$	$\color{green}2/2$ $\color{green}+100.00%$	$\color{green}180/180$ $\color{green}+100.00%$

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

[Planeshifter]

This was already implemented (the issue just wasn't closed). Notice that the changes you made here are also not applicable for base stats packages, where we do not use the tryRequire/polyfill pattern.