Implements a 2D canvas (#matrix-rain) behind the Three.js renderer
with classic Matrix green katakana/numeral/hex characters falling
in columns. The Three.js renderer is made alpha-transparent so the
rain shows through, creating an atmospheric depth effect.
- Add matrixCanvas (id=matrix-rain) appended before the WebGL canvas
- Animate at ~20 fps with a semi-transparent fade trail (0.05 alpha)
- Bright (#aaffaa) head character, dimmed by trail accumulation
- Renderer switched to alpha:true + setClearColor(0,0) — transparent bg
- scene.background removed; body background (#000008) shows through
- CSS: #matrix-rain { z-index: 0; opacity: 0.18 } for subtle blend
Fixes#262
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
