Harden trim integration

internal/trim/artifact.go

@@ -31,6 +31,11 @@ type ApplyArtifactResult struct {
 
 // ParseArtifactJSON parses and validates a serialized seriatim output artifact.
 func ParseArtifactJSON(data []byte) (Artifact, error) {
+	var decoded any
+	if err := json.Unmarshal(data, &decoded); err != nil {
+		return Artifact{}, fmt.Errorf("input JSON is malformed: %w", err)
+	}
+
 	var full schema.Transcript
 	if err := json.Unmarshal(data, &full); err == nil {
 		if err := schema.ValidateTranscript(full); err == nil {