docs: refresh the README banner artwork and add the dark diagrams

[suguanYang]

Summary

• Swaps the README banner to the new 3200x1600 exports (2x of the previous
  1600x800, same composition: lockup top left, headline bottom left, illustration
  right).
• Adds dark variants for the last light-only figures: the Vision + Text section and
  the Step 2 section. Both now switch through <picture>, so every figure in the
  README follows the reader color scheme.
• Follow-up to docs: finish the README dark-mode assets #438, which merged as 11b9dd7 with the banner scale fix before
  these two commits landed.

Verification

• The two 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). No
  image sets a height attribute, so nothing can pick up the stretch fixed in docs: finish the README dark-mode assets #438.
  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