feat(ablate): input-length bucketing + auto-instance sizing

- Add sim.input_length_{min,max} (+ CLI overrides) that drop requests
  outside the bucket after trace load, enabling per-bucket ablation
  (e.g. 0-40k) without rewriting the trace file. Applied uniformly in
  both `run`/`ablate` driver path and `oracle` analysis.

- Add cache_score_strong router (alpha=1, beta=1) to isolate how much
  of cache_affinity's win is reproducible by just retuning beta in the
  existing cache_score framework (no rendezvous, no meta-store bonus).

- Add --auto-instances to ablate: sweeps --auto-candidates ascending
  with --auto-probe-router and picks the smallest cluster size whose
  TTFT mean <= --auto-target-ttft-mean. Per-candidate calibration
  results are persisted under runs/<output_dir>/auto_instances/ so the
  pick is auditable; the chosen N is then used for the whole ablation.

src/cluster/cluster.rs

@@ -243,6 +243,8 @@ mod tests {
                 output_dir: String::new(),
                 sample_interval_s: 0.0,
                 seed: 7,
+                input_length_min: None,
+                input_length_max: None,
             },
         }
     }