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/two-pointers/0042-trapping-rain-water.org

@@ -1,18 +1,53 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0042. Trapping Rain Water :hard:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0042. Trapping Rain Water][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0042. Trapping Rain Water][0042. Trapping Rain Water]]
 :END:
 
+Given ~n~ non-negative integers representing an elevation map where the width of each bar is ~1~, compute how much water it can trap after raining.
+
+*Example 1:*
+
+
+#+begin_src
+Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
+Output: 6
+Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: height = [4,2,0,3,2,5]
+Output: 9
+#+end_src
+
+
+*Constraints:*
+
+ - ~n == height.length~
+
+ - ~1 <= n <= 2 * 10^{4}~
+
+ - ~0 <= height[i] <= 10^{5}~
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def trap(self, height: List[int]) -> int:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    int trap(vector<int>& height) {
+        
+    }
+};
 #+end_src