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

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+#+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).

org/study_deck_02/dsa/binary-search/sorting-destroys-indices.org

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+#+ANKI_DECK: study_deck_02
+* Two Sum gotcha: sorting destroys original indices :cpp:two-sum:binary-search:retrieval::recognition:
+:PROPERTIES:
+:END:
+
+** Front
+If you sort ~nums~ in-place for Two Sum, what goes wrong?
+
+** Back
+Sorting changes the indices. If the problem requires returning *original* indices, store ~{value, original_index}~ pairs before sorting.

org/study_deck_02/dsa/binary-search/std-binary-search.org

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+#+ANKI_DECK: study_deck_02
+* std::binary_search: check element exists :cpp:binary-search:algorithm:retrieval::recognition:
+:PROPERTIES:
+:END:
+
+** Front
+What does ~std::binary_search~ return, and what must the range be?
+
+** Back
+Returns ~bool~ (true if element found). The range must be *sorted*.
+
+#+begin_src cpp
+bool found = std::binary_search(vec.begin(), vec.end(), value);
+#+end_src

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

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+#+ANKI_DECK: study_deck_02
+* std::lower_bound: find position of element :cpp:binary-search:algorithm:retrieval::recognition:
+:PROPERTIES:
+:END:
+
+** Front
+What does ~std::lower_bound~ return?
+
+** Back
+An iterator to the first element *>=* the given value. Returns ~end()~ if no such element exists.
+
+#+begin_src cpp
+auto it = std::lower_bound(vec.begin(), vec.end(), value);
+#+end_src

org/study_deck_02/dsa/binary-search/two-sum-lower-bound.org

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+#+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