Function unpack_record 

pub fn unpack_record<'a>(
    spec: &ExpertRecordSpec,
    buf: &'a [u8],
) -> Result<(ExpertRecordHeader, [ProjView<'a>; 3])>

Parse a record buf (>= spec.raw_bytes()), returning the header and borrowed views of each projection’s sub-buffers. Validates the header magic and version; returns an error on any mismatch.