feature: bundle built-in fonts and expand golden fixture coverage

[yuanbohan]

Summary

• add built-in big and slant fonts and move standard into the shared resources directory
• update README examples, bundled font documentation, and CI badge for GitHub Actions
• expand golden fixtures and package only the files needed for the published crate

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test