Typed non-empty carrier for fetcher-registration bodies

Backlog stub. Spun out of R166 (graphqlschemavisitor-driven-emission, retired into R279) to keep a parked micro-refactor alive; originally surfaced as R165 (fetcher-registration-empty-body-filter) and flagged by the principles-architect read on R165 as the natural endpoint its Optional<CodeBlock> deferral should not lose track of.

FetcherRegistrationsEmitter.emit returns a Map<String, CodeBlock> whose values must be non-empty by producer convention: an empty body is the exact drift that produced R165’s bug (an empty-bodied keyset entry the registration emitter kept, ObjectTypeGenerator skipped emitting the method for, and GraphitronSchemaClassGenerator still emitted the call site against, failing the consumer’s javac). The invariant lives in a comment and a convention, not the type. Lift it into the signature: either a NonEmpty<CodeBlock> wrapper (compact-constructor checked) or a dedicated FetcherBodies record that carries the non-empty guarantee, so every consumer reads a value that cannot be empty and the "empty body" drift class is a compile error rather than a runtime/javac failure.

Independent of R279: R279 removes the reachability drift (only reachable, classified things reach emitters), but the non-empty-body invariant on this specific emitter return is an orthogonal type-system strengthening that stands on its own. Small, contained; no Spec fork beyond the wrapper-vs-record choice and the consumer-update surface (ObjectTypeGenerator is the other reader).