Function make_model_dispatcher 

pub fn make_model_dispatcher<T: ModelRuntime + 'static>() -> RoleDispatcher
where
    <T as ModelRuntime>::Error: Display,

Build a RoleDispatcher for a concrete impl of crate::roles::ModelRuntime. Used by Engine::register_*_dispatcher chain (Node::with_<role>(&value)) so single-role components dispatch through the same TypeId-keyed registry as multi-role / Protocol-bearing components.