Introduce a `Wirespec.Shape` interface (extends `Model`) that carries
the `validate(): List<String>` contract, leaving `Model` as a plain
marker. Type structs now implement `Shape`; `Enum` and `Refined` extend
`Model` directly. Tests and emitter fixtures updated to match.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>