Parallelize directory scanning and file processing

[script3r]

Refactor the directory scanner to a parallel producer-consumer architecture using ignore and rayon to dramatically improve performance.

Inspired by ripgrep, this change replaces the naive single-threaded walk with a parallel directory producer and concurrent file processing consumers. This architecture achieves ~4.37 GiB/s throughput by respecting .gitignore files, pre-filtering, and saturating CPU cores.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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