[toreleon]

💡 What: Replaced three sequential .reduce() calls on turnRows with a single indexed for loop in src/agent/backtestRunner.ts.
🎯 Why: In the backtest runner, it iterates over turnRows three times to calculate total cost, in tokens, and out tokens. This causes unnecessary O(N*K) iteration complexity and allocation overhead, especially on large backtest datasets where performance is critical.
📊 Impact: Reduces iteration overhead for these calculations by 66% (from three passes to one) and eliminates the function call overhead of .reduce() callbacks, which speeds up backtest completion.
🔬 Measurement: Verified that all unit tests (pnpm test) still pass, including orchestrator and backtest runner logic, and that pnpm typecheck correctly validates the loop typings.

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PR created automatically by Jules for task 18336003156188674368 started by @toreleon

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