⚡ Bolt: Prevent task serialization in concurrent scanners

[acebytes]

💡 What: Converted CacheScanner and NodeModulesScanner from actor to struct.
🎯 Why: In Swift structured concurrency, launching a withTaskGroup from inside an actor causes all child tasks to run sequentially on the actor's executor, destroying intended parallelism for filesystem scanning. Since these scanner types are entirely stateless, they don't need actor isolation.
📊 Impact: Significant reduction in scan time (especially for recursive node_modules searches across multiple roots) by allowing true parallel disk I/O on available CPU threads.
🔬 Measurement: Run the app and observe the completion time of a full scan. Time will be significantly shorter on systems with deep/complex Node project directories.

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PR created automatically by Jules for task 14855552840901043264 started by @acebytes

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