docs: finish the README dark-mode assets

[suguanYang]

Summary

• Fixes the banner scale. GitHub's markdown CSS sets max-width: 100% on images
  but never height: auto, so the height="500" attribute kept the box at 500px
  while the width shrank to the README column: the artwork and its headline
  rendered 768x500 instead of 768x384, a 30% vertical stretch. The attribute is
  gone and no image in the file sets a height anymore.
• Swaps the banner to the new 3200x1600 exports (2x of the previous 1600x800, same
  composition).
• Adds dark variants for the last light-only figures: the Vision + Text section and
  the Step 2 section. Every figure in the README now follows the reader color
  scheme through <picture>.

Verification

• Reproduced the height 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 of the artwork in a live page
  screenshot agreed with the stretched case: 0.48 horizontal scale against 0.58
  vertical.
• The two new dark figures are mapped by content, not by export order. OCR of the
  checked-in files: the parsing pair reads "Two tracks. One navigable memory.", the
  retrieval pair reads "Any agent. One memory contract."
• Every local asset referenced by README.md resolves, and each dark/light pair
  shares one canvas (banner 3200x1600, diagrams 3200x1280, benchmark 3200x2240).
  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 move again: knowhere-banner-light.png and
  knowhere-banner-dark.png become .jpg at the same base names, and two
  -dark.png figures are added. External pages hot-linking the old PNG banner
  paths would break.
• Rollback is reverting the commits.

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