- Fix SELL logic: correctly reduce position.quantity after selling (core bug)
Previously positions were never decremented, causing sold positions to be
settled again at backtest end, inflating final balance.
- Fix settleRemainingPositions: profitLoss now correctly computes
settlementValue - cost (was incorrectly using settlementValue.negate())
- Fix max drawdown calculation: use current balance instead of previous
iteration's runningBalance
- Frontend: add comment noting chart shows cash balance, not total equity