Function encode_metadata_only 

pub fn encode_metadata_only<T, M, E>(
    msg: &CuStampedData<T, M>,
    encoder: &mut E,
) -> Result<(), EncodeError>
where
    T: CuMsgPayload,
    M: Metadata,
    E: Encoder,

Write a metadata-only record for a stamped message: presence tag = 0u8 (no payload), followed by tov and metadata. Wire-compatible with the existing decode path — a reader sees payload: None for the frame, same as if the source had been disabled entirely, but the surrounding timestamp/status are preserved.

Codegen emits a call to this helper when a slot’s producing task is configured with HandleContent::None or HandleContent::TouchedOnly and the handle wasn’t touched.