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 0.7+.

Adds examples/visualize.rs that wires it all up: NSGA-II on Schaffer
N.1, observer closure recording per-generation hypervolume, two SVGs
written to disk.

Cargo.toml

@@ -1,3 +1,6 @@
+[workspace]
+members = [".", "heuropt-plot"]
+
 [package]
 name = "heuropt"
 version = "0.7.0"
@@ -30,6 +33,7 @@ tracing = { version = "0.1", optional = true, default-features = false, features
 
 [dev-dependencies]
 gungraun = "0.18"
+heuropt-plot = { path = "heuropt-plot" }
 proptest = "1"
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "time"] }