[Bug]: archive extraction strips special permission bits

## I have done the following

- [x] I have searched the existing issues
- [x] 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:

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

On current `main`, the regression fails with:

```text
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

```text
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.
```

## Code of Conduct

- [x] I agree to follow this project's Code of Conduct

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.

Written by the indexing model from the issue text.