Derive Macro StreamState 

#[derive(StreamState)]
{
    // Attributes available to this derive:
    #[stream_state]
}

Derive StreamStateCodec (bincode-backed) and supply the encode_state forwarder for ProducerState / ExchangeState.

Adds an inherent __vgi_encode_state(&self) helper that the trait impl can call from encode_state in one line. Phase 4 of the macro pass will land this; this stub keeps the public symbol stable.