feat: add Graph Epidemic Simulator - SIR/SIS/SEIR disease spread on networks

Interactive epidemic simulation with:
- 3 models: SIR, SIS, SEIR (with exposed phase)
- 8 network presets (Karate Club, Small World, Scale-Free, Grid, etc.)
- Configurable infection rate (beta), recovery rate (gamma), incubation rate (sigma)
- Real-time R0 calculation and epidemic curve visualization
- Interventions: vaccination (4 strategies), quarantine, contact tracing
- Autonomous advisor monitors R0 and recommends interventions
- Interactive: drag nodes, pan/zoom, double-click to infect/quarantine
- JSON and CSV export of simulation data
- Canvas-based network visualization with state-colored nodes

Usage: Open docs/epidemic.html, pick a preset, adjust parameters, click Start.

Author: sauravbhattacharya001