Infrastructure: SciPy/R validation harness & accuracy CI

[armanrasta]

Summary

Automated cross-validation against SciPy and R reference implementations. Required for Milestone 0 quality gate.

Scripts

• scripts/generate_scipy_refs.py — generate reference values from SciPy
• scripts/generate_r_refs.R — generate reference values from R
• scripts/check_accuracy.py — compare Rust output vs references

CI integration

• Run accuracy checks on every PR touching statscore-special or statscore-distributions
• Tolerances: PDF/PMF 1e-12, CDF 1e-10, PPF 1e-8, p-values 1e-6

Done when

• Special functions validated to 1e-12 on CI
• Reference files committed or generated as CI artifacts