The three runnable examples called out in spec §18.5 / §19. All open with `use heuropt::prelude::*;` so they double as a check that the prelude is sufficient on its own: - toy_nsga2.rs: Schaffer N.1 solved with NSGA-II. - random_search.rs: 2D sphere solved with RandomSearch. - custom_optimizer.rs: a minimal hill-climber implementing `Optimizer<P>` directly, demonstrating spec §2.3.