Ensure Unix homedir fallback and writability

[themuffinator]

Summary

• add Unix helpers to resolve the home directory via $HOME, passwd entries, XDG directories, and a final /tmp fallback
• prepare homedir values by creating directories and ensuring they are writable before using them
• update Sys_Init to initialize homedir defaults with the new fallback logic

Testing

• not run (not requested)

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Codex Task