feat: support transparent PNG rendering and CLI background color selection

[techwizrd]

Summary

• add transparent background support to PNG rendering via RenderOptions.background_color
• add CLI --background-color support, including transparent as a named color
• add regression coverage for transparent and default opaque PNG output

Testing

• cargo test -p ratex-types color
• cargo test -p ratex-render

[erweixin]

Thanks for the PR! Adding background_color to the rendering options is a great improvement!

[techwizrd]

Thanks!