Cat modes

CatMode is 'service' | 'api_candidate' | 'api'. It is stored on each RegistryEntry as mode.

Values

Mode	Typical source	Meaning
`service`	Class or function without Express `req`/`res` shape	Domain / application RPC.
`api_candidate`	Handler in an Express pipeline before the final handler, or `cat` with `req`+`res` but no explicit `route`	Treated as middleware in a pipeline until `registerCatPipeline` marks the last handler as `api`.
`api`	Final pipeline handler, or `cat(..., { route })`	Express endpoint; `ApiReturn` labels attach to this `fnKey`.

RPC and class instances

For style: 'class', executeRPC resolves the instance in order: explicit registerInstance, cached auto-singleton, or new Service() when the constructor was registered via @Cat / CatClass. If no instance is available and the method is not callable, the error label is NO_INSTANCE.

For style: 'function', the registered function is invoked directly—no instance step.

See Registration and RPC pipeline.