Finish the domain pipeline cleanup

internal/framework/contracts/artifact.go

@@ -41,6 +41,10 @@ type ArtifactCodec[T any] interface {
 	Kind() ArtifactKind
 	Schema() ArtifactSchema
 	MediaType() string
+	// EncodeCandidate serializes a stage result before semantic validation. It
+	// must not apply validity checks owned by typed validators; Encode remains
+	// the strict final-artifact boundary used after validation succeeds.
+	EncodeCandidate(T) ([]byte, error)
 	Encode(T) ([]byte, error)
 	Decode([]byte) (T, error)
 }