Add analytic Hessian bandwidth selection scripts

[soodoku]

Summary

• add analytic gradient and Hessian implementations for Gaussian and Epanechnikov kernels
• implement Newton–Armijo optimizer for KDE bandwidth using analytic derivatives
• provide analogous Nadaraya–Watson optimizer with analytic Hessian and simple CLI entry points

Testing

• python src/kde_analytic_hessian.py --kernel gauss --h0 1.0 (fails: ModuleNotFoundError: No module named 'numpy')
• python src/nw_analytic_hessian.py --kernel gauss --h0 1.0 (fails: ModuleNotFoundError: No module named 'numpy')
• python -m pip install numpy (fails: Could not find a version that satisfies the requirement numpy)

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https://chatgpt.com/codex/tasks/task_e_68a22c9bcd74832fa4f445d29326ecc1

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