Function resolve_font_entry 

pub fn resolve_font_entry(entry: &FontEntry) -> Result<Vec<PathBuf>>

Validate a FontEntry and resolve it to a list of TTF file paths.

• Local entry (path set, source absent): returns [path] as-is.
• Google Fonts entry (source = "google", path absent): downloads (or reads from cache) and returns one path per requested weight.
• Conflict (source and path both set): returns an error.
• Neither (path absent and source absent): returns an error.