Add candidate artifact codec decoding

internal/framework/contracts/artifact.go

@@ -50,6 +50,13 @@ type ArtifactCodec[T any] interface {
 	Decode([]byte) (T, error)
 }
 
+// CandidateArtifactCodec extends an artifact codec with strict representation
+// decoding for values that have not yet passed semantic validation.
+type CandidateArtifactCodec[T any] interface {
+	ArtifactCodec[T]
+	DecodeCandidate([]byte) (T, error)
+}
+
 // DigestArtifactSchema returns the SHA-256 digest of the exact JSON Schema
 // bytes. Schema formatting is therefore part of the registered identity.
 func DigestArtifactSchema(schema ArtifactSchema) string {