docs: give the README figures a dark counterpart

[suguanYang]

Summary

• The README now serves light and dark artwork through <picture> elements with
  prefers-color-scheme, so dark-mode readers no longer get white canvases and the
  switch needs no CSS.
• Figures covered: the top banner, the parsing and retrieval diagrams, and the
  benchmark grid.
• The banner is centered like the rest of the header, and it carries the headline
  itself: the h1 below it repeated the words already drawn inside the artwork, so
  the h1 is gone and the wording lives on in the banner alt text.
• The banner no longer carries a height attribute. GitHub's markdown CSS sets
  max-width: 100% but never height: auto, so the attribute kept the box at
  500px while the width shrank to the README column, stretching the artwork and its
  headline by 30%.
• Adds the benchmark SVG sources under docs/assets/source/, with the palette
  mapping and the render recipe documented next to them.
• Removes the four figures the current README no longer references.
• Documentation only: no API, worker, deployment, or migration impact.

Verification

• Banner scale: reproduced GitHub's rule locally at a 768px column.
  width="1000" height="500" renders 768x500 (ratio 1.536 against the natural
  2.000); width="1000" alone renders 768x384 (ratio 2.000). A template match
  against the artwork in a live page screenshot agreed with the stretched case
  (0.48 horizontal scale, 0.58 vertical), so the attribute is dropped and every
  image in the file now sets width only.
• Banner content: the light and dark exports OCR to the same lockup and
  headline; the alt text carries the headline that used to be the h1.
• Benchmark light: rendered from the design SVG at 1600x1120 with
  --force-device-scale-factor=2 (headless Chromium), which yields 3200x2240.
  Compared against the design export: edge correlation 0.79 with mean absolute
  pixel difference 1.26 (the design export is JPEG), and identical OCR values for
  all six metrics.
• Benchmark dark: derived from the same SVG by remapping six color literals;
  geometry, labels, and numbers are untouched. Color-region pixel counts match the
  light render within 0.7% (background 6,342,136 vs 6,343,012; bars 484,352 vs
  484,288; accent 136,273 vs 136,170; ink 82,545 vs 82,007), the edge correlation
  between the two renders is 0.9963, and every label still OCRs on the dark render.
  Text contrast stays at or above 4.5:1 (primary 17.8:1, secondary 5.5:1, accent
  6.7:1).
• Every local asset referenced by README.md resolves, no script or workflow reads
  the removed h1, and git diff --check is clean.
• Not tested here: the rendered README on github.com, which needs this PR's
  preview.

Deployment Notes

• No environment variables, migrations, queue, or storage changes.
• Image URLs change: knowhere-banner.png and benchmark.png are gone. External
  pages that hot-link those two paths would break; the self-hosted and dashboard
  repos were checked and do not reference them.
• Rollback is reverting the commit; the removed images stay in git history.

Checklist

• Tests were added or updated when behavior changed
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change