Package fonts defines the font preferences consumed by PDF-producing backends.
The package lives outside of both `config` and `backend` to avoid an import
cycle: backends import fonts to render PDFs with sensible defaults, while
config embeds fonts.Config to let users override those defaults.
Default returns the platform-appropriate font set.
Values are chosen to be present out-of-the-box on each OS: PT Serif and
Menlo ship with macOS; DejaVu is installed by default on most Linux
distributions; Times New Roman and Consolas are standard on Windows.