A middleware with its config pre-applied, ready to be passed to pipeline.
Carries its contributions as a record phantom — key-to-type for every key it puts on ctx — so pipeline can accumulate them onto the handler's ctx with no manual annotation. A middleware authored with defineMiddleware contributes a single key and so has a single-entry record; a composite built with defineComposite carries one entry per part.
SingleKeyEntry spells the one-key case without the braces.