[N-06] Node독립 Phase3-B: SQLite3 DB C 포팅

완료긴급🏷 node독립,phase3,SQLite3,DB,C포팅

runtime/db.c 신규 작성. fl_db_open/query/exec/close. db-query/db-exec dispatch. CREATE→INSERT→SELECT 왕복 검증. gcc -Wall -Wextra 경고 0.