refactor: move DSA notes to org/study_deck_02/

Restructure NeetCode study materials into a dedicated Anki-exportable
deck directory. Separates DSA problem notes from C++ flashcards.

- Move roadmap.org from leetcode/out/ to org/study_deck_02/
- Move 200 note files from org/cpp/dsa/ to org/study_deck_02/dsa/
- Add #+PROPERTY: STUDY_DECK_02 to all org files for org-anki export
- Simplify roadmap format: properties drawer replaces nested headings
- Update NEETCODE backlinks in all note files
- Update extract.mjs and scaffold-notes.mjs for new paths
- Create org/study_deck_02/AGENTS.md with study workflow docs
- Update root and leetcode AGENTS.md

org/study_deck_02/dsa/udfs.org

@@ -0,0 +1,42 @@
+#+title: Udfs
+#+PROPERTY: STUDY_DECK_02
+* impl
+#+begin_src cpp
+#include <vector>
+#include <numeric>
+#include <algorithm>
+class Ufs {
+  private:
+  std::vector<int> p;
+  std::vector<int> s;
+  std::vector<int> r;
+  public:
+  Ufds(int n) {
+    p.resize(n);
+    std::iota(p.begin(), p.end(), 0);
+    s.assign(n, 1); // test on equality with the assign fill and iota explain the difference
+    std::fill(r.begin(),r.end(),0);
+    numSets = n;
+  }
+  ~Ufds() {}
+  ~Ufds() = default;
+  int find(int x) {
+    if (p[x] == x) return x;
+    return p[x] = find(p[x]);
+  }
+  int find(int x) {
+    int px = p[x];
+    if (px != x) {
+      px = find(px);
+    }
+    p[x] = px;
+    return px;
+  }
+};
+#+end_src
+
+should ask what new returns? delete? what happens? why are we deleting it ? heap allocated?
+should show equivalent version of using std::array to vector
+should ask what resize does?
+
+what about a true dynammic version where we create it upon calling find()