[API] Inventory the effective public and extension API

[lrnv]

Scope

Inventory the API that Copulas.jl exposes today before deciding what should be stabilized. This is an evidence-gathering task, not authorization for API churn.

Inventory

• exported names and documented public-but-unexported names;
• methods inherited from or extending Distributions.jl;
• copula-specific operations (condition, subsetdims, Rosenblatt transforms, fitting, dependence measures, and related helpers);
• constructor forms, argument conventions, return shapes, and exceptions;
• generic fallbacks versus required family methods;
• developer extension hooks and strictly internal helpers;
• obsolete compatibility paths, unreachable methods, and accidental effective API.

For each operation, record its actual availability across representative regular, singular, conditional, high-dimensional, and fittable models. Note where availability currently depends on ad hoc test predicates or undocumented dispatch details.

Deliverable

Produce a table classifying every relevant name or operation as:

1. stable public API candidate;
2. capability-dependent public API candidate;
3. documented extension point;
4. internal implementation detail;
5. compatibility-sensitive legacy or accidental surface;
6. dead/unreachable candidate supported by evidence.

Acceptance criteria

• the inventory covers exports, documentation, implementation, and tests;
• current behavior is recorded without assuming the documentation is correct;
• generic fallback and specialization responsibilities are distinguished;
• compatibility-sensitive findings become focused follow-ups;
• no speculative removal or rename is bundled into the audit.

Parent: #427. Related housekeeping program: #422.