Generate backtest results output (JSON + CSV)

Save backtest results in two formats:

• JSON: full result including config used, summary metrics, and trade log
• CSV: one row per trade with columns: pair_id, entry_time, exit_time, direction, entry_spread, exit_spread, gross_pnl, fees, net_pnl, holding_seconds
• Save equity curve as separate CSV (timestamp, equity_value)
• Include metadata: backtest run timestamp, parameters used, data date range