docs: add graph classes gap analysis

Add comprehensive gap analysis document identifying 200+ missing
graph classes across 6 major categories:

- Forbidden subgraph classes (120+)
- Intersection graph classes (50+)
- Probe graph classes (30+)
- Width parameter classes (20+)
- Perfect graph variants (20+)
- Other structural classes (40+)

Document includes current coverage analysis, implementation
priorities, and architectural recommendations for scaling
GraphBox to support 200+ graph classes with automated code
generation.

Author: Mearman