add flashcard generation tooling and binary search cards

- gen-flashcards.py: auto-generate recognition cards from all problem files
- toolkit/gen-problem-cards.org: 199 auto-generated problem cards
- 5 binary search tool cards (std::binary_search, std::lower_bound, comparison, two-sum pattern, sorting gotcha)
- two-sum.org: add binary search C++ attempt
- lc-org.el: add doom emacs localleader keybinding support

org/study_deck_02/dsa/binary-search/lower-bound-vs-binary-search.org

@@ -0,0 +1,11 @@
+#+ANKI_DECK: study_deck_02
+* When to use lower_bound vs binary_search :cpp:binary-search:algorithm:retrieval::recognition:
+:PROPERTIES:
+:END:
+
+** Front
+When should you use ~std::lower_bound~ instead of ~std::binary_search~?
+
+** Back
+Use ~lower_bound~ when you need the *position/index* of the element.
+Use ~binary_search~ when you only need to know *if it exists* (bool).