Function decode_template42_for_file_by_message 

pub fn decode_template42_for_file_by_message(
    path: &Path,
    width: usize,
    height: usize,
) -> Result<Vec<Option<Vec<f32>>>, String>

High-level helper: read Section7 payloads and Section5 template42 params for the file, then batch decode template42 messages and return per-message decoded arrays (or None).