Function prepare_encrypted_object 

pub fn prepare_encrypted_object<T: Serialize>(
    block_size: usize,
    crypto: &dyn CryptoEngine,
    codec: &PostcardCodec,
    kind: ObjectKind,
    obj: &T,
) -> FsResult<Vec<u8>>

Serialize, encrypt, and pad a typed object into a block-sized buffer (without writing to store). Returns the ready-to-write block bytes.