enhancementgood first issue
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描述
Add numx_stats_correlation (Pearson's r) to the stats module, operating on two equal-length arrays.
Follow CONTRIBUTING.md exactly: full Doxygen header, NULL-checks, numx_status_t return, all four deliverables (header, impl, test, docs page). Use Kahan (compensated) summation for the accumulation, matching the pattern already used in numx_vec_dot (src/linalg.c) after #57.