pub fn single_currency_for_market(trd_market: Option<i32>) -> Option<i32>
单币种账户的默认 view currency (对齐 C++ INNData_Trd_CommonCurrency.cpp::GetTrdMarketCurrency line 63-87)
INNData_Trd_CommonCurrency.cpp::GetTrdMarketCurrency
单币种账户只支持这一个 currency, 用户传别的 → reject.