Define model registry, suitability matrix and validation requirements #86

Parent Feature: #43
Parent roadmap: #28

Scope

Create a canonical registry for available generator models/profiles and their intended uses.

Acceptance criteria

• every model declares version, RNG/reproducibility contract and supported configuration;
• suitability/limitations are explicit;
• required validation metrics are listed before a model is marked stable;
• default-model changes require compatibility review.