Add multi-font auto-detect tests and CI workflow

- Add 8 open-source test fonts (Roboto, Noto Sans JP/KR/CJK SC,
  WenQuanYi Micro Hei, Alibaba PuHuiTi, OPPO Sans, MiSans)
- Refactor multi-font tests to auto-detect supported language groups
  by probing font cmap, eliminating manual coverage declarations
- Add CI workflow with build, test, clippy, and fmt checks
- Update dependabot from pip to cargo + github-actions