enhancementgood first issue
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描述
Add numx_stats_skewness and numx_stats_kurtosis to the stats module.
numx_stats_variance already uses Welford's online algorithm for numerical stability, see the note in #59 and #57's PR about why naive summation is avoided in this codebase, follow the same care here (an online algorithm extending Welford's, e.g. tracking higher moments incrementally, is preferable to a naive sum-based formula).
Follow CONTRIBUTING.md exactly: full Doxygen header, NULL-checks, numx_status_t return, all four deliverables (header, impl, test, docs page).