feat: scene graph - node tree + transform inheritance

- SceneNode: position/rotation(scale) + parent/child hierarchy
- World matrix: local = translate * rotate * scale, world = parent.world * local
- Euler/Quaternion rotation support
- Dirty flag propagation for efficient updates
- Scene init/find/create/destroy operations

CMakeLists.txt

@@ -24,6 +24,7 @@ set(ENGINE_SOURCES
     Engine/Rendering/mesh.c
     Engine/Rendering/texture.c
     Engine/Rendering/camera.c
+    Engine/Scene/scene_graph.c
     Engine/ECS/world.c
     Engine/Input/input.c
     Engine/Physics/physics.c