documentation

Write or update a concise Markdown document describing a CppModel plant model and the controller it exercises - signal interface, plant behaviour as per-cycle update laws (TeX math), parameters and timing, the controller's states and transitions, the closed loop, the simplifications made and why, and how it is validated. Starts as a draft from the user's own initial description before any code exists, stays untouched while the solution changes, and is reshaped to match the code once the plant and controller are settled. Use when asked to document a plant model, a controller, or a simulation, at the start of cppmodel:plant-model or cppmodel:decouple-component (to collect the initial description), and when a solution is settled or a refinement is promoted.