refactor: Move recipe generator to cook/ directory and fix flattening bugs

- Move recipe-generator/ to cook/ for cleaner structure

- Fix ksflatten-relative path conversion to handle all %include paths

- Fix validation exit code to only fail on actual errors

- All recipes now generate and flatten successfully via make all

cook/ingredients/validation/success-if-empty.ks

@@ -0,0 +1,6 @@
+# Validation fragment - success if empty
+
+%post
+# If this is being run, it's a success
+echo SUCCESS > /root/RESULT
+%end