[Core] Paper trading engine — simulated order fill and portfolio tracking

[jackby03]

Summary

There is no paper trading mode. Users cannot test strategies without real money. This is a critical safety feature for any trading platform.

What needs to be done

PaperTradingEngine in Omnijure.Core

• Implements IExchangeClient so it is a drop-in replacement
• Maintains a virtual Portfolio with initial balance (configurable, default $10,000 USDT)
• On PlaceOrderAsync:
  • Market order: fills immediately at the last known candle close price
  • Limit order: queues the order and fills when price crosses the limit level
• Tracks open orders, filled orders, P&L per trade, total return %
• Emits OrderFilledEvent via IEventBus just like the real client would
• Exposes PaperPortfolio: Balance, Positions, OpenOrders, TradeHistory

Mode switching

• AppSettings.General gets a TradingMode enum: Live | Paper
• IExchangeClientFactory returns either BinanceClient or PaperTradingEngine based on the setting

Acceptance criteria

• PaperTradingEngine implements IExchangeClient
• Market orders fill at current price on next tick
• Limit orders queue and fill when price is reached
• PaperPortfolio tracks balance, positions, PnL correctly
• Mode can be toggled from settings without restart
• Unit tests: place → fill → assert portfolio state

References

• src/Omnijure.Core/Entities/Exchange/
• Issue [Exchange] Define and implement order placement on IExchangeClient #1 (OrderRequest / OrderResult models)