Function engine_positions_to_portfolio_balance 

pub fn engine_positions_to_portfolio_balance(
    positions: &HashMap<(WalletAddress, String), EnginePosition>,
    wallet: &WalletAddress,
    reference_prices: &HashMap<String, f64>,
) -> PortfolioBalance

Convert engine positions to a PortfolioBalance for margin calculations.

This is a runtime helper that uses crate-local types (PortfolioBalance, PositionData) which cannot live in the pure hypercall-engine crate.