Implements Three.js Points-based particle system for rain and snow.
- Rain: 3000 particles, fast vertical fall, blue-white tint
- Snow: 1500 particles, slow drift with x-axis sway, white
- Cycle button: ⛅ Weather → 🌧 Rain → ❄ Snow → repeat
- Full scene setup (renderer, camera, lighting, grid floor)
- Particles reset to top when they fall below y=0
Fixes#117