LodExtensionAccess<T> extension

Dart extension providing convenient access to LOD functionality.

This extension adds a lod getter to NodeFlowController that lazily registers and returns the LodExtension. This pattern keeps the controller lean while providing ergonomic access:

// Access LOD state reactively
Observer(builder: (_) {
  if (controller.lod.showPorts) {
    return PortsLayer(...);
  }
  return const SizedBox.shrink();
});