Replace hero with monochrome particle artwork

[hello-malek]

What changed

• replaced the soft, colored JPEG hero with a lossless 3200 × 1280 grayscale PNG
• adopted the selected monumental crop composition with one high-density particle orb
• updated the README to reference the new asset
• added a deterministic Swift renderer so the artwork can be reproduced precisely

Why

The previous hero looked low-resolution and introduced color and promotional UI elements that did not fit Aither's desired monochrome identity. The new artwork stays sharp at Retina sizes and keeps the presentation focused on the particle orb itself.

Validation

• verified 3200 × 1280 pixel dimensions
• verified PNG output in grayscale color space
• rerendered to a temporary file and confirmed byte-for-byte deterministic output
• ran git diff --check