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.

examples/visualize.rs

@@ -0,0 +1,53 @@
+//! Visualize an NSGA-II run on Schaffer N.1 with the `heuropt-plot`
+//! companion crate — produces a `pareto_front.svg` of the final
+//! Pareto front.
+//!
+//! Run with: `cargo run --release --example visualize`
+
+use heuropt::prelude::*;
+use heuropt_plot::pareto_front_svg;
+
+struct Schaffer;
+
+impl Problem for Schaffer {
+    type Decision = Vec<f64>;
+    fn objectives(&self) -> ObjectiveSpace {
+        ObjectiveSpace::new(vec![Objective::minimize("f1"), Objective::minimize("f2")])
+    }
+    fn evaluate(&self, x: &Vec<f64>) -> Evaluation {
+        Evaluation::new(vec![x[0] * x[0], (x[0] - 2.0).powi(2)])
+    }
+}
+
+fn main() {
+    let problem = Schaffer;
+    let bounds = vec![(-5.0_f64, 5.0_f64)];
+    let space = problem.objectives();
+
+    let mut opt = Nsga2::new(
+        Nsga2Config {
+            population_size: 50,
+            generations: 100,
+            seed: 42,
+        },
+        RealBounds::new(bounds.clone()),
+        CompositeVariation {
+            crossover: SimulatedBinaryCrossover::new(bounds.clone(), 15.0, 0.5),
+            mutation: PolynomialMutation::new(bounds, 20.0, 1.0),
+        },
+    );
+
+    let result = opt.run(&problem);
+
+    let svg = pareto_front_svg(
+        &result.pareto_front,
+        &space,
+        700,
+        450,
+        "NSGA-II on Schaffer N.1 — final Pareto front",
+    );
+    std::fs::write("pareto_front.svg", svg).expect("write pareto_front.svg");
+
+    println!("Final front size: {}", result.pareto_front.len());
+    println!("Wrote pareto_front.svg");
+}