yazi: add palette doc image + regeneration script for kanagawa-dragon

Document the kanagawa-dragon flavor's colors in a single rendered image,
generated from the theme's own source files so it stays accurate.

- scripts/render-yazi-palette.ts: bun script that parses flavor.toml
  (palette names + usage scan) and tmtheme.xml (minimal plist parser),
  then renders a labeled swatch grid via ImageMagick to
  _themes/kanagawa-dragon/palette.png. Re-run: `bun scripts/render-yazi-palette.ts`.
- palette.png: UI theme (18 colors), tmtheme editor (6) and syntax
  scopes (21 unique colors), each labeled with name/hex/usage.

Author: kriswill