Creating a standalone sandbox particle and physics simulator as you described is a big task, but I'll outline a solid structure using JavaScript and libraries like PIXI.js and PlayCanvas. Below is a conceptual design, modular architecture, and some code snippets to help you get started.
### Project Structure
```
/sandbox-particle-simulator
├── index.html
├── styles.css
├── app.js
├── /js
│ ├── emitter.js
│


