Merge pull request #8 from CausalInferenceLab/feat/fdc

feat(fdc): When Confounders Hide | Front-Door Criterion 챕터 추가

Author: Funbucket

.gitignore

@@ -20,6 +20,9 @@ videos/*/preview/
 # Prompts (skills replaced runtime usage)
 /prompts
 
+# Large data files (cached locally, not tracked in git)
+book/fdc/data/
+
 # OS
 .DS_Store