Regular Polygon
RegularPolygon(center, n_sides, radius) is a 2D primitive: an n_sides-gon
inscribed in a circle of radius around center. It expands to a Polygon
of N lines and composes with Extrusion, Lathe, Loft, and Sweep.
Hexagons stay available as the Hexagon(center, radius) shorthand;
RegularPolygon is the generalization for any side count.