Function load_config_with_override 

pub fn load_config_with_override(
    explicit: Option<PathBuf>,
) -> Result<(HarnessConfig, ConfigSource), String>

load_config with an explicit config path (e.g. from --config) taking precedence over $AV_CONFIG and the search paths. The explicit path must exist — a dangling operator-supplied path is a hard error, never a silent fallback to a different config.