function u = ctrl_null(t, x, plant, ref)
% CTRL_NULL  No-op controller.  Returns u = 0.
%
%  Use for unforced-plant simulations (feedback-only response to Q_sg) or
%  as a baseline to compare against closed-loop modes.
%
%  Signature matches all other controllers:
%     u = ctrl_<mode>(t, x, plant, ref)
%  so it can be dropped into pke_solver as a function handle.

    u = 0;

end