Five fully interactive lessons are implemented in `src/engine/visualizers/`:
1. JavaScript Loop Visualizer
- Animates array elements as a `for` loop iterates
- Shows index, sum, and condition variables updating in real time
- Color-coded loop phases: init, check, body, increment, done
- SVG loop flow diagram with animated active phase
2. HTTP Request Lifecycle
- Sequence diagram: Mobile App → Django → PostgreSQL → response
- Animated message arrows between actors