Add convex_polygon shape; wraps Voronoi cells into the shape pipeline (#114 follow-up)

[petlenz]

Summary

Adds a 2D convex_polygon shape that bridges voronoi_generator_2d's raw vertex output to the standard rvegen pipeline (writers, generators, intersection dispatch, etc.). After this PR a polycrystal-style RVE is two calls:

```cpp
rvegen::voronoi_generator_2d gen{1.0, 1.0, seeds};
auto cells = gen.build();
std::vector<std::unique_ptr<rvegen::shape_base>> shapes;
for (auto& v : cells) {
shapes.push_back(std::make_unique<rvegen::convex_polygon>(std::move(v)));
}
// shapes now flows through voxel_writer / gmsh_geo_writer / svg_writer.
```

What lands

• convex_polygon<T> shape type, registered via static_indexing<convex_polygon<T>, shape_base<T>>.
• Full shape_base contract:
  • area() — shoelace formula (abs of signed area, robust to winding).
  • volume() — 0 (2D shape).
  • is_inside(point) — left-of-every-edge test (O(N), correct for convex polygons only).
  • make_bounding_box() — AABB over vertices, returns rectangle_bounding.
  • get_middle_point() / set_middle_point() — area-weighted centroid; translation moves every vertex.
  • clone() returns an independent copy.

Design notes

• Convex-only by design. is_inside is the simple "same side of every edge" test, valid only for convex polygons. The motivating input (Voronoi cells) is always convex. A general-polygon shape with ray-cast is_inside would be a separate type.
• Winding-order convention: CCW. The ctor trusts the caller; voronoi_generator_2d already emits CCW. A CW polygon would pass area() (which uses abs) but make is_inside reject every query — diagnosed easily by a test that constructs an obviously-inside point.
• Translation is O(N). set_middle_point moves every vertex by the centroid delta. For a typical Voronoi cell with 4–8 vertices this is cheap; future "thousands of vertices" cases would benefit from an offset-vector indirection (cf. mesh_inclusion's pattern from Add mesh_inclusion_input: JSON-driven STL placement (#9 phase 2) #27), but that's premature for current use cases.

Test plan

• Unit square: area = 1, centroid = (0.5, 0.5), interior point inside, exterior outside, edge point inside.
• Right triangle: area = 0.5, interior point inside, exterior point outside.
• set_middle_point({10, 5, 0}) translates the polygon; original interior point now exterior, new centroid interior.
• AABB matches min/max of vertex coordinates.
• clone() produces an independent copy (translating the copy doesn't affect the original).
• < 3 vertices throws.
• End-to-end Voronoi wrapping: 2-seed Voronoi → 2 convex_polygon shapes, each correctly classifies its seed's neighbourhood.
• All 15 ctest targets green locally.

Deferred

• Wrap-helper voronoi_to_shapes(generator) that returns the shape_vector directly — trivial composition but a separate ergonomics call.
• Python binding for ConvexPolygon.
• Output-writer support: voxel_writer already works via is_inside; gmsh_geo_writer's write_2d would need a Plane Surface dispatch case to emit the polygon natively (today it skips with (unsupported 2D shape skipped)). That's a separate gmsh-writer extension.