Add reproducible contour-fitting benchmark cases

[Jayktopus]

Goal

Make line, circle, arc and B-spline changes measurable against known geometry.

Initial cases

• rectangle and regular polygon;
• washer with known inner and outer diameters;
• rounded slot;
• concave wrench-like profile;
• mixed line and arc contour.

Acceptance criteria

• Ground truth is generated independently of the algorithm under test.
• Metrics include edge error in pixels and millimetres, topology and CAD element count.
• Tests are deterministic.
• Fixtures and their licences are documented.