⚡ Bolt: Prevent serialisation bottleneck by converting Scanner actors to structs

[acebytes]

💡 What: Converted CacheScanner and NodeModulesScanner from actor to struct and added explicit performance documentation.

🎯 Why: Using an actor to manage a withTaskGroup where child tasks invoke blocking methods on self inadvertently serializes the tasks on the actor's context, destroying parallelism. Since these scanners are stateless components that only interact with thread-safe dependencies (FileManager.default), actor isolation is unnecessary. Changing them to structs allows the recursive filesystem traversals and size calculations to run truly concurrently across multiple threads.

📊 Impact: Drastically reduces scan time for deep hierarchies and large file systems by unlocking true hardware parallelism.

🔬 Measurement: Verify by running the application and observing the time taken to complete a full scan of all cache categories and node_modules directories; the duration should be noticeably shorter, particularly on systems with many files.

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PR created automatically by Jules for task 6970769507282085271 started by @acebytes

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