⚡ Bolt: Fix serialized actor bottlenecks in parallel scanners

[acebytes]

💡 What: Converted CacheScanner and NodeModulesScanner from actor to struct and made their internal methods synchronous without actor isolation.

🎯 Why: While both scanners used withTaskGroup to attempt parallel directory traversal across multiple CPU cores, their actor isolation forced the child tasks to jump back to the actor's serialized execution context to perform the actual synchronous file I/O. This effectively serialized all the disk scanning operations, running them one by one. Since neither scanner maintains mutable state and FileManager.default is thread-safe for these operations, converting to struct allows the child tasks to truly run in parallel.

📊 Impact: Significantly improves scanning speed, especially for developers with many node_modules folders or large caches, by utilizing all available CPU cores for disk traversal rather than serializing them on a single actor.

🔬 Measurement: Profile the application before and after. Notice that CPU core utilization spikes to 100% across multiple cores during a scan, whereas previously it would be constrained. Scanning large directories like ~/.npm and recursive node_modules will complete visibly faster.

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PR created automatically by Jules for task 6796066008954764646 started by @acebytes

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