Derive Macro StateGraph 

#[derive(StateGraph)]
{
    // Attributes available to this derive:
    #[channel]
}

Derive macro for StateGraph state types.

Annotate fields with #[channel(reducer = "fn_name")] to specify a reducer function for that channel. Fields without the attribute use LastValue (default).

Robustness Check: This macro enforces that every field must have #[serde(default)] (or be an Option which handles missing keys gracefully). This prevents silent state loss during graph resume operations.