Function write_encrypted_object 

pub fn write_encrypted_object<T: Serialize>(
    store: &dyn BlockStore,
    crypto: &dyn CryptoEngine,
    codec: &PostcardCodec,
    block_id: u64,
    kind: ObjectKind,
    obj: &T,
) -> FsResult<()>

High-level helper: serialize a typed object, encrypt it, pad to block size, and write.