Support extension-owned schema artifact providers

[jwils]

Why

Disk-backed artifacts and in-memory schema results expose different JSON APIs. Adding each ingestion format's methods to core would require changes there for every third-party format.

What

Add an extension-owned artifact registry shared by disk and in-memory results. JSON and protobuf provide their own artifact APIs through this registry.

How

Extensions register a name, factory, require path, and configuration during schema definition. Runtime metadata preserves that registration. Each factory supplies disk and in-memory providers with the same extension-specific interface.

schema_artifacts.extension_artifacts.fetch(name) loads and caches only the requested provider. Core defines no JSON or protobuf artifact methods. A custom-provider test exercises registration, artifact generation, disk loading, configuration, and coexistence with another format. Other tests cover absent providers, duplicate registration, lazy loading, and instance isolation.

Risk

Callers of JSON methods on FromDisk or Results must use extension_artifacts.fetch("json") first. Existing artifacts must be regenerated to include provider registrations. Applications need the provider gems they access; unrelated providers remain unloaded.

Testing

No manual testing. Local datastore tests remain blocked by Docker Desktop's required organization sign-in.

Bigger picture

Stacked on #1384. Third-party gems can now expose custom artifact APIs without adding format-specific methods or dependencies to core.