Function config_path_from 

pub fn config_path_from(
    xdg_config_home: Option<&str>,
    home: Option<&str>,
    appdata: Option<&str>,
    is_windows: bool,
) -> Option<PathBuf>

Derive bhtune’s config file location from raw environment values rather than reading std::env directly – keeps discovery fully unit-testable across every permutation without mutating real process environment variables.

• Windows (is_windows = true): %APPDATA%\bhtune\bhtune.toml.
• Elsewhere: $XDG_CONFIG_HOME/bhtune/bhtune.toml, falling back to $HOME/.config/bhtune/bhtune.toml.