binance: preserve exchangeInfo symbol precision fields

[mtshit]

Summary

• add Binance symbol-level precision fields to mkt-types::MarketInfo
• populate base_asset_precision, quote_precision, and quote_asset_precision from spot exchangeInfo
• add a conversion test to ensure the precision fields are preserved

Why

The Binance spot exchangeInfo payload already exposes symbol precision metadata, but MarketInfo currently drops it. Downstream execution code cannot consistently reason about order precision without access to these fields.

Testing

• cargo test --release -p mkt-exchange-binance -p mkt-types