Function encode_record 

pub fn encode_record(
    operation: SyncdOperation,
    index: &[u8],
    value: &SyncActionValue,
    keys: &ExpandedAppStateKeys,
    key_id: &[u8],
    iv: &[u8; 16],
) -> (SyncdMutation, Vec<u8>)

Encode and encrypt a mutation into a SyncdRecord.

This is the reverse of decode_record — it takes plaintext data and produces an encrypted record ready for sending.

Returns

A tuple of (SyncdMutation, value_mac_bytes) where value_mac is needed for hash state updates and persistence.