fix: change directory attributes detection and set external attributes

[Its-Just-Nans]

fix for #737

As mentioned in #737. An issue was introduced in 072abec (#471). This commit is trying to detect symlink for System::Dos but when we use add_symlink we force the System::Unix.

and fix #433

[amazon-q-developer]

Review Summary

This PR modifies the directory attributes detection logic in src/types.rs. While the reorganization of constants and improved documentation are good, there is a critical logic error in the MS-DOS symlink detection that needs to be addressed.

Critical Issue

The new symlink check in the System::Dos branch incorrectly interprets the high 16 bits of external_attributes as Unix mode. According to the ZIP specification, for DOS-system archives, only the low 16 bits are meaningful (MS-DOS attributes), and the high 16 bits should not be used for Unix-style mode detection. This could cause incorrect symlink identification.

Recommendations

Before merging, please:

1. Review the symlink detection logic for DOS system archives
2. Verify the behavior with test cases for DOS archives with and without the symlink bits accidentally set in the high 16 bits
3. Ensure the changes align with APPNOTE.TXT section 4.4.15 regarding DOS compatibility

The PR is marked as WIP, so please address this issue before requesting final review.

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[gemini-code-assist]

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