feat(ui): introduce ColumnLayout widget and refactor widget parenting

- Add ColumnLayout widget that arranges children vertically with spacing.
- Refactor Widget::add_child to automatically pass `this` as parent.
- Update DebugCollector to use ColumnLayout for consistent spacing.
- Expose children() accessor in Widget for layout management.

include/Cubed/debug_collector.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include "Cubed/ui/column_layout.hpp"
 #include "Cubed/ui/label.hpp"
 #include "Cubed/ui/widget.hpp"
 
@@ -18,7 +19,7 @@ public:
     bool handle_event(const Event& e);
 
 private:
-    Widget m_widget;
+    ColumnLayout m_widget;
     std::unordered_map<std::string, Label*> m_component;
 };
 

include/Cubed/ui/column_layout.hpp

@@ -0,0 +1,24 @@
+#pragma once
+#include "Cubed/ui/widget.hpp"
+namespace Cubed {
+class ColumnLayout : public Widget {
+public:
+    ColumnLayout(const ColumnLayout&) = delete;
+    ColumnLayout(ColumnLayout&&) = delete;
+    ColumnLayout& operator=(const ColumnLayout&) = delete;
+    ColumnLayout& operator=(ColumnLayout&&) = delete;
+    ColumnLayout(Widget* parent);
+    ~ColumnLayout();
+
+    void update(float dt) override;
+
+    void set_spacing(int spacing);
+    // No need for parent node pointer; do not modify children's anchors and
+    // scale.
+
+    void layout();
+
+private:
+    int m_spacing = 0;
+};
+} // namespace Cubed

include/Cubed/ui/widget.hpp

@@ -32,16 +32,13 @@ public:
     virtual bool handle_mouse_move_event(const MouseMoveEvent& e);
 
     template <typename T, typename... Args> T& add_child(Args&&... args) {
-        auto widget = std::make_unique<T>(std::forward<Args>(args)...);
+        auto widget = std::make_unique<T>(std::forward<Args>(args)..., this);
         T& ref = *widget;
         m_children.emplace_back(std::move(widget));
         return ref;
     };
 
 protected:
-    virtual void on_update(float dt);
-    virtual void on_render(Renderer& renderer);
-    virtual glm::vec2 compute_position() const;
     Widget* m_parent = nullptr;
     std::string m_id;
     float m_window_height = 0;
@@ -50,6 +47,14 @@ protected:
     Anchor m_anchor = Anchor::TOP_LEFT;
     glm::ivec2 m_offset{0, 0};
 
+    std::vector<std::unique_ptr<Widget>>& children();
+
+    const std::vector<std::unique_ptr<Widget>>& children() const;
+
+    virtual void on_update(float dt);
+    virtual void on_render(Renderer& renderer);
+    virtual glm::vec2 compute_position() const;
+
 private:
     std::vector<std::unique_ptr<Widget>> m_children;
 };