Function resolve_arch 

fn resolve_arch<S: BuildHasher>(
    metadata: &HashMap<String, String, S>,
    architecture: Option<&str>,
) -> String

Resolve the GGUF key prefix: the caller’s architecture when it knows one, else the file’s own general.architecture, normalised for lookup. Empty when neither is available, which is harmless — lookup_raw’s unprefixed fallback still runs.