## 목적\\\\\\\\n현재 C codegen이 지원하는 stdlib 함수가 runtime.c 기반으로 제한됨.\\\\\\\\n가장 많이 쓰이는 3개 우선 포팅.\\\\\\\\n\\\\\\\\n## 작업\\\\\\\\n1. runtime.c에 fl_file_read, fl_file_write 구현\\\\\\\\n2. fl_println 개선 (int/float/string/nil 분기)\\\\\\\\n3. fl_str_concat 가변 인자 지원\\\\\\\\n\\\\\\\\n## 통과 기준\\\\\\\\n- test_runtime.c 단위 테스트 PASS\\\\\\\\n- wc -l runtime/runtime.c < 600