feat: wire bucket identities through driver outputs

src/sim/events.rs

@@ -1,5 +1,6 @@
 //! Event types for the discrete-event engine.
 
+use crate::router::BucketId;
 use crate::types::{InstanceId, ReqId};
 
 #[derive(Debug)]
@@ -7,7 +8,10 @@ pub enum Event {
     /// New trace request arrives at the cluster router.
     Arrival { req_id: ReqId },
     /// Per-instance scheduler tick (continuous batching).
-    BatchTick { instance: InstanceId },
+    BatchTick {
+        bucket: BucketId,
+        instance: InstanceId,
+    },
     /// Periodic time-series sample of all instances.
     Sample,
     /// Stop the simulation early (used internally).