feat: populate note files with problem descriptions and code stubs

Add populate-notes.mjs that fetches problem descriptions and
Python/C++ code stubs from LeetCode's GraphQL API. Populated
all 197 NeetCode 150 note files with:
- Problem description (examples, constraints)
- Python code stub (function signature)
- C++ code stub (function signature + includes)

API responses cached in leetcode/.cache/leetcode/ for instant re-runs.

org/study_deck_02/dsa/bit-manipulation/0260-single-number-iii.org

@@ -1,18 +1,64 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0260. Single Number III :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0260. Single Number III][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0260. Single Number III][0260. Single Number III]]
 :END:
 
+Given an integer array ~nums~, in which exactly two elements appear only once and all the other elements appear exactly twice. Find the two elements that appear only once. You can return the answer in *any order*.
+
+You must write an algorithm that runs in linear runtime complexity and uses only constant extra space.
+
+*Example 1:*
+
+
+#+begin_src
+Input: nums = [1,2,1,3,2,5]
+Output: [3,5]
+Explanation:  [5, 3] is also a valid answer.
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: nums = [-1,0]
+Output: [-1,0]
+#+end_src
+
+
+*Example 3:*
+
+
+#+begin_src
+Input: nums = [0,1]
+Output: [1,0]
+#+end_src
+
+
+*Constraints:*
+
+ - ~2 <= nums.length <= 3 * 10^{4}~
+
+ - ~-2^{31} <= nums[i] <= 2^{31} - 1~
+
+ - Each integer in ~nums~ will appear twice, only two integers will appear once.
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def singleNumber(self, nums: List[int]) -> List[int]:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    vector<int> singleNumber(vector<int>& nums) {
+        
+    }
+};
 #+end_src