Function resolve_input_source 

pub fn resolve_input_source(
    explicit: Option<&Path>,
    config: &Config,
) -> Result<InputSource, String>

Resolve the <PLAYSET_PATH> argument shared by check, merge, graph, and simplify. When the user supplies an explicit path it is used verbatim; otherwise <paradox_data_path>/dlc_load.json (the launcher’s currently-active playset) is the implicit default. Returns a human-readable error string when neither source produces a usable path.