refactor: change extra_field from Arc<Vec<u8>> to Arc<[u8]>

[lovasoa]

ZipFileData currently stores extra_field and central_extra_field as Option<Arc<Vec<u8>>>. This requires two pointer dereferences to reach the data (Arc ptr → Vec ptr → heap buffer), and the Vec's 3-word header (ptr, len, capacity) wastes space since the data is never resized after construction.

This changes the fields to Option<Arc<[u8]>>, which stores the byte slice inline in the Arc allocation. One indirection instead of two, and one usize saved per field (no capacity).

The AES extra data mutation path (update_aes_extra_data) can no longer use Arc::make_mut since [u8] is unsized. Instead it copies to a Vec, mutates, and converts back. I think it's acceptable since it only runs once during AES encryption setup on the write path.

This is a breaking change since the fields are exposed publicly

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request optimizes the storage of extra data fields within ZipFileData by changing their underlying type from Arc<Vec<u8>> to Arc<[u8]>. This modification enhances performance by reducing pointer dereferences and improves memory usage by eliminating the overhead of Vec's internal structure. The change primarily affects how these byte slices are managed and accessed, with a minor adjustment to a specific mutation path for AES extra data.

Highlights

• Memory Optimization: Refactored extra_field and central_extra_field in ZipFileData from Option<Arc<Vec<u8>>> to Option<Arc<[u8]>> to improve memory efficiency.
• Performance Improvement: Reduced pointer dereferences from two to one for accessing extra field data, leading to better performance.
• AES Extra Data Handling: Adjusted the update_aes_extra_data function to temporarily convert Arc<[u8]> to Vec<u8> for mutation, then back to Arc<[u8]>, as Arc::make_mut is not applicable to unsized types.

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