⚡ Bolt: Optimize backtest subset generation via bounded loops

[toreleon]

💡 What: Optimized the ticker extraction logic in src/agent/backtestRunner.ts by replacing the previous .map(), spread Set allocations, Array.from(), and .slice() chain with explicit for loops that terminate as soon as maxCandidates is reached.
🎯 Why: Creating intermediate mapped arrays, concatenating via spread, putting all items into a Set, converting back to Array, and slicing it forces full iteration over potentially large candidate lists and creates unnecessary garbage collection overhead in the very hot backtest loop.
📊 Impact: Replaces multiple O(N) full-array iterations and intermediate structure allocations with a bounded O(K) lookup where K <= maxCandidates, leading to significantly lower GC pressure and faster candidate extraction during each backtest step.
🔬 Measurement: Code execution successfully passes all validation tests and pnpm test and pnpm typecheck correctly execute without regressions.

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PR created automatically by Jules for task 7583512746036927747 started by @toreleon

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