[Housekeeping] Reduce maintenance and CI costs

[lrnv]

Objective

Run a staged housekeeping program to minimize Copulas.jl maintenance and CI costs without reducing functional coverage or mixing unrelated behavioral changes into a single large pull request.

The guiding principle is one source of truth per mathematical or numerical mechanism. Similar-looking implementations must first be classified as accidental duplication, justified specialization, or genuinely different numerical problems.

Working rules

• Measure before changing, especially compilation time versus warm execution time.
• Keep mechanical moves separate from numerical or API changes.
• Add characterization tests before consolidating numerical implementations.
• Preserve closed forms and justified multiple-dispatch specializations.
• Organize tests around distinct dispatch paths, not every family-operation Cartesian product.
• Use small reviewable PRs, each with an explicit maintenance or CI benefit.

Success indicators

• fewer duplicated numerical implementations;
• clear ownership and discoverability of internal types;
• explicit mapping between representative models and dispatch paths;
• lower CI wall time and/or compilation work with equivalent functional coverage;
• removal of genuinely dead internal code and accidental API surface;
• documented follow-ups where consolidation is unsafe or premature.

The native GitHub sub-issues attached below track the independent workstreams. This issue is for coordination and aggregate progress only.