feat: add permission checking via set_user/clear_user

[Koan-Bot]

Summary

• Adds opt-in Unix permission checking for mocked file operations via set_user($uid, @gids) / clear_user()
• When active, operations check owner/group/other permission bits against the mock identity

Why

Without permission checks, Test::MockFile always allows all operations regardless of file mode bits. This makes it impossible to test code that depends on permission-based behavior (e.g., read-only files, restricted directories, privilege checks).

How

• _check_perms($mock, $access) checks permission bits using the standard Unix model (owner/group/other triads)
• _check_parent_perms($path, $access) checks parent directory permissions for ops that modify directory contents
• Root (uid 0) bypasses read/write checks; execute still requires at least one x bit
• Permission checks added to: open, sysopen, opendir, unlink, mkdir, rmdir, chmod, chown
• chown uses mock identity instead of $> / $) when set_user is active
• Fully backward compatible: without set_user(), no checks occur

Testing

• 16 subtests in t/perms.t covering: basic set/clear lifecycle, owner/group/other read/write, root bypass, sysopen, opendir, parent dir checks (unlink/mkdir/rmdir), chmod ownership, chown identity, non-existent files, multiple groups, file creation parent checks

Closes #3

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