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.

src/ui/column_layout.cpp

@@ -0,0 +1,24 @@
+#include "Cubed/ui/column_layout.hpp"
+
+namespace Cubed {
+ColumnLayout::ColumnLayout(Widget* parent) : Widget(parent) {}
+ColumnLayout::~ColumnLayout() {}
+
+void ColumnLayout::update(float dt) {
+    Widget::update(dt);
+    layout();
+}
+
+void ColumnLayout::set_spacing(int spacing) { m_spacing = spacing; }
+
+void ColumnLayout::layout() {
+    auto& children = Widget::children();
+    int y = 0;
+    for (auto& child : children) {
+        child->set_offset({0, y});
+        child->set_anchor(Anchor::TOP_LEFT);
+        y += child->height() + m_spacing;
+    }
+}
+
+} // namespace Cubed