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.