Add Rust and C++ linked list implementations with build/debug configs

Rust version in src/main.rs (Cargo project) and a C++ port in cpp/main.cpp
using raw pointers, both with VS Code build tasks and gdb-based debug
configs. Vendors vscode-debug-visualizer and visualization as submodules
for reference while building a custom node-table graph visualizer.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

cpp/main.cpp

@@ -0,0 +1,100 @@
+#include <iostream>
+#include <optional>
+#include <sstream>
+
+// A single node containing data and a raw pointer to the next node
+template <typename T>
+struct Node {
+    T data;
+    Node<T>* next;
+};
+
+// The main wrapper for the linked list tracking the head node
+template <typename T>
+class LinkedList {
+public:
+    LinkedList() : head_(nullptr) {}
+
+    ~LinkedList() {
+        Node<T>* current = head_;
+        while (current) {
+            Node<T>* next = current->next;
+            delete current;
+            current = next;
+        }
+    }
+
+    // Raw-pointer ownership can't be safely duplicated by a shallow copy
+    LinkedList(const LinkedList&) = delete;
+    LinkedList& operator=(const LinkedList&) = delete;
+
+    // Add a new element to the front of the list
+    void push_front(T data) {
+        head_ = new Node<T>{std::move(data), head_};
+    }
+
+    // Remove and return the front element of the list
+    std::optional<T> pop_front() {
+        if (!head_) {
+            return std::nullopt;
+        }
+        Node<T>* old_head = head_;
+        T data = std::move(old_head->data);
+        head_ = old_head->next;
+        delete old_head;
+        return data;
+    }
+
+    // Read the front element without removing it
+    const T* peek_front() const {
+        return head_ ? &head_->data : nullptr;
+    }
+
+    // Build a JSON string describing the list as a graph, for the VS Code
+    // "Debug Visualizer" extension: watch `list.toDebugGraph()` while paused.
+    // Schema: https://hediet.github.io/visualization/docs/visualization-data-schema.json
+    std::string toDebugGraph() const {
+        std::ostringstream json;
+        json << "{\"kind\":{\"graph\":true},\"nodes\":[";
+        for (Node<T>* n = head_; n; n = n->next) {
+            if (n != head_) json << ",";
+            json << "{\"id\":\"" << n << "\",\"label\":\"" << n->data << "\"}";
+        }
+        json << "],\"edges\":[";
+        for (Node<T>* n = head_; n && n->next; n = n->next) {
+            if (n != head_) json << ",";
+            json << "{\"from\":\"" << n << "\",\"to\":\"" << n->next << "\"}";
+        }
+        json << "]}";
+        return json.str();
+    }
+
+private:
+    Node<T>* head_;
+};
+
+// Force instantiation of every member, including toDebugGraph(), which
+// nothing in this file calls directly -- without this, gdb has no symbol
+// to invoke from the debugger's watch expression.
+template class LinkedList<int>;
+
+int main() {
+    LinkedList<int> list;
+
+    // Demonstrate pushing items
+    list.push_front(10);
+    list.push_front(20);
+    list.push_front(30);
+
+    // Demonstrate peeking at the top item
+    if (const int* top = list.peek_front()) {
+        std::cout << "Top element: " << *top << "\n"; // Output: 30
+    }
+
+    // Demonstrate popping items
+    while (auto value = list.pop_front()) {
+        std::cout << "Popped: " << *value << "\n";
+    }
+
+    return 0;
+}