⚡ Bolt: [performance improvement] Fix actor serialization to enable true parallel scanning

[acebytes]

💡 What: Changed CacheScanner and NodeModulesScanner from actor to struct.

🎯 Why: In Swift structured concurrency, using an actor to manage a withTaskGroup where tasks invoke methods directly on the same actor inadvertently serializes the tasks. This prevents true parallel execution, bottlenecking performance. Since these scanners are stateless and rely on thread-safe dependencies (like FileManager.default), they do not need actor isolation.

📊 Impact: Unlocks true parallel scanning across all threads, significantly reducing the total scan time, especially when many cache categories or deep directory trees are involved.

🔬 Measurement: Measure the total execution time of CacheoutViewModel.scan() before and after this change. A notable decrease in scan duration is expected.

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PR created automatically by Jules for task 7692733786188521692 started by @acebytes

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