feat(heuropt-plot): v0.1.0 — SVG visualization companion crate

Adds heuropt-plot, a tiny SVG-only plotter that takes heuropt
results and emits scatter plots (pareto_front_svg) and line plots
(convergence_svg). No heavy 'plotters' or 'tiny-skia' dep — hand-
rolled SVG so the crate adds <100 KB to a build.

Workspace setup: root Cargo.toml gains [workspace] with members =
['.', 'heuropt-plot']. heuropt-plot has its own version (0.1.0) and
publishes independently against heuropt 0.8+.

Adds examples/visualize.rs that wires it up: NSGA-II on Schaffer
N.1, plain run() (no observer plumbing), final-front SVG written to
disk.

src/prelude.rs

@@ -4,12 +4,12 @@
 //! use heuropt::prelude::*;
 //! ```
 
+#[cfg(feature = "async")]
+pub use crate::core::async_problem::AsyncProblem;
 pub use crate::core::{
     Candidate, Direction, Evaluation, Objective, ObjectiveSpace, OptimizationResult,
     PartialProblem, Population, Problem, Rng, rng_from_seed,
 };
-#[cfg(feature = "async")]
-pub use crate::core::async_problem::AsyncProblem;
 
 pub use crate::traits::{Initializer, Optimizer, Repair, Variation};