fix: guard legacy runtime paths for bucketed configs

tests/smoke.rs

@@ -6,7 +6,7 @@ use std::io::Write;
 
 use kvcache_simulator::config::*;
 use kvcache_simulator::driver;
-use kvcache_simulator::replay::ReplayEvictPolicy;
+use kvcache_simulator::replay::{self, PlacementEntry, ReplayEvictPolicy};
 
 fn base_config(trace_path: &str, out_dir: &str, mode: RouterMode) -> Config {
     Config {
@@ -70,6 +70,26 @@ fn base_config(trace_path: &str, out_dir: &str, mode: RouterMode) -> Config {
     }
 }
 
+fn bucketed_config(trace_path: &str, out_dir: &str, mode: RouterMode) -> Config {
+    let mut cfg = base_config(trace_path, out_dir, mode);
+    cfg.cluster.num_instances = None;
+    cfg.cluster.buckets = vec![
+        BucketConfig {
+            name: "short".into(),
+            input_length_min: 0,
+            input_length_max: 64,
+            num_instances: 2,
+        },
+        BucketConfig {
+            name: "long".into(),
+            input_length_min: 65,
+            input_length_max: 128,
+            num_instances: 1,
+        },
+    ];
+    cfg
+}
+
 fn write_synthetic_trace(path: &std::path::Path) {
     // 5 distinct conversations, each with 8 turns. Within a conversation,
     // turn k+1 reuses the prefix of turn k (shared first ~10 blocks) and
@@ -274,3 +294,36 @@ fn ablation_parallel_matches_serial() {
         assert!((lhs.miss_rate - rhs.miss_rate).abs() < 1e-12);
     }
 }
+
+#[test]
+fn bucketed_configs_are_rejected_by_legacy_runtime_paths() {
+    let tmp = std::env::temp_dir().join("kvcache_sim_bucketed_reject");
+    let _ = std::fs::remove_dir_all(&tmp);
+    std::fs::create_dir_all(&tmp).unwrap();
+    let trace_path = tmp.join("trace.jsonl");
+    write_synthetic_trace(&trace_path);
+
+    let cfg = bucketed_config(
+        trace_path.to_str().unwrap(),
+        tmp.to_str().unwrap(),
+        RouterMode::Random,
+    );
+
+    let result = driver::run(&cfg, Some("bucketed_guard"));
+    assert!(result.is_err(), "bucketed run should fail");
+    let err = result.err().unwrap();
+    assert!(err.to_string().contains("cluster.buckets"));
+
+    let err = driver::ablate_fixed_placement(&cfg, &[RouterMode::Random], &[ReplayEvictPolicy::Lru])
+        .expect_err("bucketed ablation should fail");
+    assert!(err.to_string().contains("cluster.buckets"));
+
+    let err = replay::replay_fixed_placement(
+        &cfg,
+        &[],
+        &Vec::<PlacementEntry>::new(),
+        ReplayEvictPolicy::Lru,
+    )
+    .expect_err("bucketed replay should fail");
+    assert!(err.to_string().contains("cluster.buckets"));
+}