refactor(editor): Phase 1 architecture refactoring

- Decouple Panel from Core::Layer (P0 issue resolved)
- Add EventBus with type-safe event system
- Add ISelectionManager interface with SelectionManagerImpl
- Add IEditorContext for dependency injection
- Update EditorLayer to use new architecture
- Update Application to create and inject EditorContext

New files:
- editor/src/Core/EventBus.h
- editor/src/Core/EditorEvents.h
- editor/src/Core/ISelectionManager.h
- editor/src/Core/SelectionManagerImpl.h
- editor/src/Core/IEditorContext.h
- editor/src/Core/EditorContextImpl.h

This enables future improvements: Undo/Redo, serialization, component extensibility.

editor/src/panels/Panel.cpp

@@ -3,5 +3,11 @@
 namespace XCEngine {
 namespace Editor {
 
+Panel::Panel(const std::string& name) 
+    : m_name(name), m_isOpen(true) {
+}
+
+Panel::~Panel() = default;
+
 }
 }