Edge routing

After layout assigns node positions, edges need to be routed as concrete paths between nodes. This involves three layers of increasing sophistication.

Node boundary clipping computes where an edge line intersects the source and target node shapes, so edges terminate at shape boundaries rather than node centers. This requires intersection functions for each shape type (ellipse, rectangle, polygon). Polyline routing threads edges through the positions of virtual nodes inserted during hierarchical layout, producing multi-segment paths that follow the layered structure. Spline fitting converts polyline control points into smooth cubic Bezier curves. The `splines` graph attribute controls which mode is used. Polyline routing should be implemented first as the baseline, with spline fitting as a follow-up enhancement.

References

• Line-ellipse intersection
• Line-line intersection (Wikipedia)
• Bezier curve (Wikipedia)
• Edge routing in graph drawing (survey)

The issue names no files, tests, or entry points; start by inspecting the repository's existing layout implementation and the referenced edge-routing material. Define the polyline-routing baseline first, including node-boundary clipping, and treat spline fitting as a later enhancement; done should be demonstrated by routed paths that follow hierarchical virtual-node positions.