Node shape geometry

Each node shape needs two things: an SVG path generator (given center and size, produce the drawable path) and a boundary intersection function (given an incoming edge angle, compute where it hits the shape perimeter). These are used by the SVG renderer and edge routing respectively.

Shapes should be implemented in tiers. Tier 1 covers the most common shapes: `ellipse` (the default), `box`/`rect`, `circle`, `point`, `plaintext`/`plain`, and `none`. Tier 2 adds polygon-based shapes: `diamond`, `triangle`, `pentagon`, `hexagon`, `octagon`. Tier 3 adds `record` and `Mrecord`, which have their own label sub-language (`{a|{b|c}|d}`) requiring a mini-parser and recursive cell layout algorithm. Tier 4 covers the remaining ~40 Graphviz shapes. The `width`, `height`, and `fixedsize` attributes control shape sizing.

References

• Graphviz node shapes
• Graphviz record-based nodes
• Regular polygon (Wikipedia)
• SVG basic shapes (MDN)

Start by tracing how node shapes are consumed by the SVG renderer and edge-routing logic. Use the Graphviz node-shapes and record-node references to define the geometry, sizing attributes, and record syntax requirements. Done means the requested shape tiers, including recursive record layouts, are supported with path generation and boundary intersections.