[CORE] Implement LayerStack and Layer base class

[mrgreenapple24]

Goal

A stack of layers and overlays that receive updates and events in order.

Tasks

• Layer base class: OnAttach(), OnDetach(), OnUpdate(Timestep), OnEvent(Event&), OnImGuiRender()
• LayerStack: internal std::vector<Layer*>, layers inserted before overlays
• PushLayer(Layer*), PushOverlay(Layer*), PopLayer(), PopOverlay()
• Iterate front-to-back for OnUpdate, back-to-front for rendering
• Event::Handled flag stops propagation down the stack

Acceptance Criteria

Pushing two layers and one overlay results in correct update/event ordering verified by log output.