Function forward_mutated_frames 

async fn forward_mutated_frames(
    parser: &mut ControlStreamParser,
    data: &[u8],
    send: &mut SendStream,
    side: ProxySide,
    ctx: &ForwardCtx,
) -> Result<(), ProxyError>

Feed bytes into the capturing control parser, then forward each completed frame — giving the hook the chance to rewrite it.