feat: iOS zip compatibility mode for server-side unzippers

[cursor]

Motivation

Multiple reports of iOS-created ZIPs failing when extracted by server tools (unzipper, Java ZipInputStream) while Android-created archives from the same app work:

• iOS unzipping not working after upgrading to v7 #333 — iOS unzipping / server invalid signature after v7
• ios压缩后zip上传到无法端，无法解压 #323 — iOS zip uploads fail server-side (invalid entry size)

Local macOS unzip often succeeds, so the issue is interoperability (Zip64, data descriptors, encryption flags, how SSZipArchive writes entries), not outright corruption.

Proposal

Add a compatibility path for iOS zip creation that produces archives reliably consumed by common server unzippers:

• Prefer classic deflate without unexpected Zip64 when not needed
• Ensure entry sizes / CRCs are written in a form Java/unzipper accept
• Document any trade-offs (e.g. AES vs STANDARD)

Optionally expose a flag, e.g. zip(..., { compatibility: 'interop' }), or make interop the default for non-password zips if safe.

Acceptance criteria

• Reproduce iOS unzipping not working after upgrading to v7 #333/ios压缩后zip上传到无法端，无法解压 #323 style failure with a fixture; fix covered by a regression test or documented verification script
• iOS-created (non-password) zip extracts cleanly with Node unzipper and Java ZipInputStream
• Android behavior unchanged (already interoperable)
• README notes the compatibility guarantees

Priority

P1