Function decode_postcard_exact 

pub fn decode_postcard_exact<T: DeserializeOwned>(
    body: &[u8],
) -> Result<T, CodecError>

Deserialize a bare postcard body, rejecting surplus trailing bytes. A versioned format that exists to catch drift treats trailing garbage as corruption (e.g. a partial overwrite), not a clean record. Used by VersionedCodec::decode_version implementations per version.