Function load_registry 

pub async fn load_registry(
    pool: &Pool,
    dialect: &dyn Dialect,
    tenant_id: &str,
    package_id: &str,
    path_segment: &str,
) -> Result<ExtensibleRegistry, AppError>

Load the extensible-field registry for one entity from the KV store.

Returns an empty registry when no row exists (the feature is opt-in and must never error merely because a tenant has declared no extensible fields).