feat: add Graph Diffusion Simulator (SIR/SIS/IC models)

Interactive tool for simulating epidemic and information spreading on networks:
- 3 models: SIR (epidemic), SIS (recurring), Independent Cascade
- 7 preset graphs: Karate Club, Small World, ER Random, Scale-Free (BA), Grid, Star, Complete K8
- Real-time epidemic curve visualization (S/I/R over time)
- Proactive recommendations: super-spreader detection, vaccination targets, containment scoring
- R0 estimation, peak infection tracking
- Click nodes to set patient zero, drag to rearrange
- JSON export of simulation history
- Force-directed layout with interactive node dragging

Usage: Open docs/diffusion.html, select a preset graph, click nodes to infect, press Play.

Author: sauravbhattacharya001