assets: add social-preview banner + logo icons at 512/256/128/64

Complete visual-asset set for vibe-sci in one pass:

  assets/logo.png               — 2K square master (navy + amber quill + neural trail)
  assets/social-preview.png     — 1920x1080 16:9 banner with wordmark text
                                  ("vibe-sci" + "autonomous research paper writer"
                                   tagline in amber). Crops cleanly to GitHub's
                                   1280x640 social-card spec.
  assets/icons/logo-512.png     — Lanczos-downscaled master for README hero spots
  assets/icons/logo-256.png     — plugin marketplace thumbnail scale
  assets/icons/logo-128.png     — moderate-density list view scale
  assets/icons/logo-64.png      — tab favicon / inline-badge scale

Both the 2K logo and the banner were generated via Google Gemini 3.1
Flash Image Preview ("nano-banana-2") through a throwaway local script;
icon variants are deterministic Pillow Lanczos resamples of the same
master, so the visual identity is one-source-of-truth.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jeffhsu-chiuchau