Function fetch_icon_svg_in 

pub fn fetch_icon_svg_in(
    icon: &str,
    color: &str,
    width: u32,
    height: u32,
    cache_dir: &Path,
) -> Result<Vec<u8>>

Core of fetch_icon_svg, with the cache directory injectable so tests can exercise the cache-hit path without touching $HOME or the network — mirrors google_fonts::resolve_google_font’s cache_dir parameter.