Macro define_config_registry 

macro_rules! define_config_registry {
    ($vis:vis static $name:ident = [$($definition:expr),* $(,)?];) => { ... };
}

Builds a static ConfigRegistry from a list of ConfigDefinition items.

Product crates normally wrap this macro in their own module that names product-specific keys and default functions. Keeping registration declarative makes it easier for services to hand the same registry to default initialization, validation, OpenAPI presentation, and admin UI metadata.