Function sum_members_for 

pub fn sum_members_for(
    handler: &Handler,
    actors: &HashMap<String, ActorDecl>,
) -> Option<Vec<SumMember>>

v0.52: resolve a handler’s by clause into ordered sum members, if it names more than one actor. None for a single-actor handler (those keep the existing seam paths). The checker has already validated peer/scheme/ reachability rules; this lowers the verified members for emission.