Add the Matrix wallpaper

[vaprdev]

Matrix digital rain in the Classics collection (fixed night look): pixel-drawn mirrored katakana and digits fall in streams with pale mint heads and green trails, with a dimmer, slower layer behind. Slow by design, tuned with the owner to a quarter of the first speed.

• Calm: sparse. Lively: classic density. Teeming: dense, with code surges.
• Clicks start a stream with a ring of bright glyphs; a finished agent pours fast streams; errors stutter and glitch in red; the easter egg spells FOLLOW THE WHITE RABBIT.

Images already on assets.

Verified: typecheck, build, dev/check.ts matrix in octant and quadrant modes and --verify-dirty (0 failed, about 6 ms/frame, 11 cells of flicker at teeming), screenshot check (42 up to date), README images match assets.