feat: add ZipFileBuilder for parallelizing compression

[alex]

Compresses a file's contents independently of any ZipWriter, so that multiple files can be compressed on separate threads and then appended serially with ZipWriter::add_prepared_file.

[Copilot]

Pull request overview

Adds a new “prepare then append” workflow to support parallelizing per-file compression work outside of ZipWriter, while still producing correct ZIP metadata when appending the pre-compressed payloads.

Changes:

• Introduces ZipFileBuilder to compress a single file entry into memory while tracking CRC-32 and sizes, producing a PreparedZipFile.
• Adds ZipWriter::add_prepared_file to append a PreparedZipFile by copying its already-compressed bytes verbatim (including handling ZIP64 header patching and stream/data-descriptor mode).
• Adds tests covering roundtrip reading, duplicate-name rejection (and writer recovery), stream mode behavior, and encryption rejection.

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[Its-Just-Nans]

Hi,

Thanks for the MR, a little question in comment.

Also, what do you think about adding a test with a thread (like the documentation)?

[Its-Just-Nans]

Why not using a Box<[u8]> ?

and change line 2526

-let data = cursor.into_inner();
+let data = cursor.into_inner().into_boxed_slice();

[alex]

I originally had a test case with some threads (similar to the doc comment), but I removed it on the theory that the borrow checker meant it had to work :-) Happy to add it back.

[alex]

Addressed, thanks!

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/write.rs:1801

• add_prepared_file() writes the already-compressed payload via ZipWriter::write_all(), which unconditionally updates ZipWriterStats (CRC32 + byte count) on the compressed bytes (see ZipWriter::write at write.rs:761+). For prepared files the CRC/sizes are already known, so this does extra CPU work proportional to the compressed size and can partially defeat the purpose of off-thread compression. Writing directly to the inner writer avoids the redundant hashing and bookkeeping.

        // start_entry leaves the inner writer as a bare Storer (a compression encoder is only
        // installed by start_file*), so the already-compressed bytes pass through unchanged.
        let result = self.write_all(&data);
        self.ok_or_abort_file(result)?;

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/write.rs:2332

• Doc comment reads as if encryption-free options would fail. It likely meant that creating a builder will fail when encryption is requested; adding a comma clarifies the intended meaning.

/// Encryption is not supported; pass options without encryption or creation will fail.