⚡ Bolt: [replace O(n) filter with O(log n) lookup]

[toreleon]

💡 What: Added findLastBarIndex binary search utility in src/data/sources/dnsePublic.ts and updated src/agent/backtestRunner.ts to use it instead of .filter() for time-series lookups.
🎯 Why: Inside the heavy backtest execution loop, .filter((b) => b.time <= asOf) performed an O(n) scan over entire historical arrays and allocated temporary structures, creating an unnecessary bottleneck and GC pressure on what should be O(1) or O(log n) operations on sorted data.
📊 Impact: Reduces array allocations inside backtest loops and significantly improves loop performance from O(n) per lookup to O(log n) without sacrificing readability.
🔬 Measurement: Verify with pnpm test and pnpm typecheck. The backtest runner completes faster.

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PR created automatically by Jules for task 8921108676787207229 started by @toreleon

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