fix: close bucketed cluster config model gaps

src/router/prefix_affinity.rs

@@ -51,7 +51,7 @@ pub struct PrefixAffinityRouter {
 
 impl PrefixAffinityRouter {
     pub fn new(config: &Config) -> Self {
-        let n = config.cluster.num_instances as usize;
+        let n = config.cluster.total_instances() as usize;
         let cfg_fan = config.cluster.router.affinity_fan_out;
         // fan_out: if configured, use it; otherwise auto = max(2, n/8).
         let fan_out = if cfg_fan > 0 {