Add Zero — multiplayer voxel sandbox (Three.js TSL + Rust→WASM)

[stevekwon211]

Adds Zero to the Demos section.

• Multiplayer voxel sandbox running in the browser, free, no install, no signup
• Renderer: Three.js with TSL (WebGPU backend, WebGL fallback for unsupported browsers)
• Meshing: custom voxel engine in Rust → WebAssembly, running in a worker pool, using Dual Contouring
• Multiplayer: Cloudflare Worker + Durable Object
• Same WASM engine also ships native to macOS, Windows, and iOS

Placed after the texture-compression demo at the end of the Demos list. Happy to reposition or trim the description if you'd prefer.