benchmark: complete Covariance and Nonparametric canonical matrices

[TheHiddenObserver]

Goal

Expand canonical coverage from the current partial P2 source to the maintained Covariance and Nonparametric capability matrices.

Scope

• Covariance: empirical, shrinkage, robust, and graphical estimators where public and maintained;
• Nonparametric: KDE, kernel regression, KernelRidge/CV, KernelPCA, natural cubic splines, and additional maintained kernels;
• define aligned scales, operations, statistical metrics, external references, and synchronized timing;
• retain partial coverage explicitly until each method is complete;
• add structured sources, parser tests, manifest registration, generated assets, and coverage-matrix updates.

Parent: #100