Function plan_upstream 

pub fn plan_upstream(
    self_node_id: &str,
    class: ReplicaClass,
    requested: Option<&CascadeUpstream>,
) -> (UpstreamChoice, Option<CascadeRefusal>)

Decide where a node streams from, given its streaming class and an optionally-requested intermediate source.

The decision is total and side-effect-free:

• No requested intermediate → UpstreamChoice::Primary, no refusal.
• Requested, but the node is a voting member → refuse with CascadeRefusal::VotingMemberDirectOnly and fall back to the primary.
• Requested, but the intermediate is this node itself → refuse with CascadeRefusal::SelfReference and fall back to the primary.
• Requested, node is an async read-replica, source is another node → UpstreamChoice::Intermediate, no refusal.

Returning the refusal alongside the (safe) fallback choice lets the caller honour the connection immediately while still logging why a configured cascade did not take effect.