fix(02): reject trailing JSON after the first decoded value (WR-03)

Co-authored-by: Cursor <cursoragent@cursor.com>

tide/diff.go

@@ -130,6 +130,9 @@ func decodeJSON(raw []byte) (any, error) {
 	if err := dec.Decode(&v); err != nil {
 		return nil, err
 	}
+	if dec.More() {
+		return nil, fmt.Errorf("trailing JSON after first value")
+	}
 	return v, nil
 }