Clarify handling of UID/GID defaults in RawPOSIX

[rennergade]

In the RawPOSIX library, functions that retrieve identity-related values (e.g., UID, GID, EUID, EGID) currently return hardcoded defaults rather than querying the host system. This appears to be a remnant from the earlier RustPOSIX implementation. We should decide whether these values should continue to use defaults or be retrieved using the appropriate system calls (e.g., getuid(), getgid()). This decision will impact POSIX compliance and how identity information is represented in lind-wasm.

[JustinCappos]

You should call into the OS. In theory you would change this behavior with a grate if you wanted to. Assuming using a grate is free (which is one of the maxims we have for 3i), then this is the right architecture.