Shared trading domain primitives (Side, TimeInForce, AccountId, …)
and little-endian encoding helpers, extracted from melin-trading so
that crates which only need the wire-level data model (the protocol
codec, market-data publishers, gateway crates) don't transitively pull
the trading engine glue.
melin-trading re-exports both modules for backwards compatibility,
so existing melin_trading::types::* / melin_trading::le::* paths
keep resolving while consumers migrate.