Add Algorithm Documentation

[lowhung]

Description

Add detailed documentation for the physics simulation and layout algorithms.

Areas to Document

1. Force-directed layout (layout.rs)

   • Explain the physics model (spring-electrical model)
   • Document each force type and its purpose
   • Explain convergence behavior

2. Bezier curves (drawing.rs)

   • Explain quadratic Bezier math
   • Document tangent calculation for arrows

3. Health computation (graph.rs)

   • Document threshold meanings
   • Explain health propagation from edges to nodes

Acceptance Criteria

• Add module-level documentation to layout.rs
• Add detailed function docs for physics calculations
• Add examples in doc comments where helpful
• Run cargo doc and verify documentation renders correctly

Files Affected

• src/ui/layout.rs
• src/ui/drawing.rs
• src/graph.rs