A simulated broker: the emulator a Pine strategy trades against.
There is no order book and no exchange. A backtest replays historical bars
and asks, bar by bar, what would have happened — so filling an order is a
modelling assumption ([FillModel]), not a match against a real resting
order. Everything else — position, average price, commission, the trade log,
equity — is plain accounting that does not depend on the venue, so there is
one [BarBroker], not one per exchange.
Placing real orders is deliberately out of scope: in Pine that happens outside the strategy, when an alert is delivered to an external system. This crate only simulates.