Wallets._update_live 删除交叉保证金位置: collateral==0 过滤器在初始保证金回退之前运行

**1. `Wallets._positions` is incomplete in live cross-margin.** Consumers such as `get_collateral()`, `_check_exit_amount()` and the `/balance` RPC only see the subset of positions for which ccxt happened to derive a collateral value.