Function read_enum_header 

pub fn read_enum_header<'de, R: Reader<'de>>(
    r: &mut WxfReader<R>,
    tok: ExpressionEnum,
) -> Result<(u64, String), Error>

Read an enum list header (token already consumed): skips the head, reads the variant name string, and returns (total_arity, variant_name). The caller reads the remaining total_arity - 1 payload values.