feat: implement kernel receive timestamps on Linux/Android

[syszery]

Closes #2574.

Add Linux/Android support for kernel receive timestamps via SO_TIMESTAMPNS.

• Enable SO_TIMESTAMPNS on socket setup
• Parse SCM_TIMESTAMPNS ancillary messages
• Expose timestamp via RecvMeta::timestamp (Option<u64>, nanoseconds since Unix epoch)
• Increase CMSG_LEN from 88 to 96 bytes on Linux/Android

Tested on Linux x86_64. All tests pass locally.

[thomaseizinger]

Thanks! Looks clean.

Two comments.

[thomaseizinger]

LGTM

@djc let me know if you are happy with the change to core::time::Duration.

Commits should be squashed into one before merging. Can you do that please @syszery ? Don't worry about attributing to me for the last one.

[syszery]

Hi @thomaseizinger, thank you for the feedback and the improvements! I reviewed the changes, and I noticed that we now convert with Duration::from_secs. But I think we would need from_nanos here because the current implementation in the decoder uses nanoseconds.

However, to make the intent clearer and the code safer, I suggest to make the following change to decode directly into a Duration. This way, we avoid this entirely.

#[cfg(any(target_os = "linux", target_os = "android"))]
(libc::SOL_SOCKET, libc::SCM_TIMESTAMPNS) => {
    let ts = unsafe { cmsg::decode::<libc::timespec, libc::cmsghdr>(cmsg) };
    
    // Handle potential negative values on 32-bit/64-bit systems
    let secs = u64::try_from(ts.tv_sec).unwrap_or(0);
    let nsecs = u32::try_from(ts.tv_nsec).unwrap_or(0);

    self.timestamp = Some(core::time::Duration::new(secs, nsecs));
}

[djc]

Looks okay to me modulo a few nits. Thanks!

[syszery]

I've addressed all the feedback and squashed the history.

• Moved the Duration imports to the top level of the corresponding modules (unix.rs and tests.rs).
• Converted libc::timespec directly into a Duration in ControlMetadata::decode.
• Kept the crate::log:: prefix in unix.rs as it is the prevailing style across the rest of this file.

[thomaseizinger]

One more nit, otherwise LGTM