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/two-sum-lower-bound.org

@@ -0,0 +1,16 @@
+#+ANKI_DECK: study_deck_02
+* Task: Two Sum complement lookup with lower_bound :cpp:binary-search:two-sum:retrieval::production:
+:PROPERTIES:
+:END:
+
+** Front
+Given sorted ~nums~ and index ~i~, write C++ to check if ~target - nums[i]~ exists in the rest of the array using ~std::lower_bound~.
+
+** Back
+#+begin_src cpp
+int complement = target - nums[i];
+auto it = std::lower_bound(nums.begin() + i + 1, nums.end(), complement);
+if (it != nums.end() && *it == complement) {
+    return {(int)i, (int)std::distance(nums.begin(), it)};
+}
+#+end_src