[Bug]: archive extraction strips special permission bits

Start in ArchiveReader.setFileAttributes and inspect the ArchiveReaderTests case named preserveSpecialPermissionBits. Run swift test --disable-automatic-resolution -Xswiftc -warnings-as-errors --filter preserveSpecialPermissionBits. Done means extraction preserves the full 0o7777 POSIX permission field, including sticky and set-ID bits, while excluding unrelated file-type bits.

I have done the following

• I have searched the existing issues
• I reproduced the issue against apple/containerization main at 50f77222964ed3d01dcb53f803ea2d511656a9dc

Steps to reproduce

1. Add an ArchiveReaderTests case that writes a directory entry with mode 01777.

2. Extract the archive with ArchiveReader.

3. Read the extracted directory's .posixPermissions.

4. Run:

   swift test --disable-automatic-resolution -Xswiftc -warnings-as-errors --filter preserveSpecialPermissionBits
   

On current main, the regression fails with:

Expectation failed: ((perms & permMask) -> 511) == (0o1777 -> 1023)
Special permission bits should be preserved

The cause is ArchiveReader.setFileAttributes, which applies entry.permissions & 0o777. That mask strips the sticky, set-user-ID, and set-group-ID bits.

Current behavior

Archive extraction preserves only ordinary owner/group/other read, write, and execute permissions. A 01777 directory is extracted as 0777; set-user-ID and set-group-ID bits are also discarded.

This is a regression in main after #816 changed the extraction mask from the full mode to 0o777.

Expected behavior

Archive extraction should preserve the full POSIX permission field (0o7777) while continuing to exclude unrelated file-type bits.

Environment

• OS: macOS 26.5.1 (25F80)
• Xcode: 26.6 (17F113)
• Swift: Apple Swift 6.3.3
• Containerization source: apple/containerization main at 50f77222964ed3d01dcb53f803ea2d511656a9dc

Relevant log output

Test preserveSpecialPermissionBits() recorded an issue at ArchiveReaderTests.swift
Expectation failed: ((perms & permMask) -> 511) == (0o1777 -> 1023)
Special permission bits should be preserved
Test run with 1 test in 1 suite failed with 1 issue.

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