Fix multi-turn replay fidelity: track realized output tokens across all components

The replayer and proxy were building multi-turn prompts from trace tokens,
but the model generates different output tokens. Subsequent turns had wrong
prefix tokens, causing cache misses and invalid experimental measurements.

- replay.py: min_tokens=max_tokens for deterministic length, return_token_ids
  to capture actual output, _apply_realized_prefix for next-turn correction
- proxy: extract output token_ids from SSE, record prompt+output as realized
  prefix in shadow cache, extract _handle_local_request to deduplicate
- bench.sh/launch_elastic_p2p.sh: default elastic mode to unified policy
- mooncake_connector: only send prompt blocks (not stale output blocks),
  track failed_recving_block_ids for error recovery

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/bench.sh

@@ -26,7 +26,8 @@ TRACE="${TRACE:-$PROJECT_DIR/traces/w600_r0.0015_st30.jsonl}"
 # Defaults
 TAG=""
 MODE="baseline"     # baseline | elastic
-POLICY="linear"     # linear | lmetric
+POLICY="linear"     # linear | lmetric | unified
+POLICY_SET=false
 N_INSTANCES=8
 BASE_PORT=8000
 PROXY_PORT=9090
@@ -44,7 +45,7 @@ while [[ $# -gt 0 ]]; do
     case "$1" in
         --tag) TAG="$2"; shift 2 ;;
         --mode) MODE="$2"; shift 2 ;;
-        --policy) POLICY="$2"; shift 2 ;;
+        --policy) POLICY="$2"; POLICY_SET=true; shift 2 ;;
         --instances) N_INSTANCES="$2"; shift 2 ;;
         --requests) REQUESTS="$2"; shift 2 ;;
         --trace) TRACE="$2"; shift 2 ;;
@@ -60,11 +61,15 @@ while [[ $# -gt 0 ]]; do
 done
 
 if [ -z "$TAG" ]; then
-    echo "Usage: bench.sh --tag NAME --mode {baseline|elastic} [--instances N] [--policy {linear|lmetric}] [--requests N]"
+    echo "Usage: bench.sh --tag NAME --mode {baseline|elastic} [--instances N] [--policy {linear|lmetric|unified}] [--requests N]"
     echo "  Trace QPS is controlled by sample_trace.py --sample-ratio, not by bench.sh."
     exit 1
 fi
 
+if [ "$MODE" = "elastic" ] && [ "$POLICY_SET" = "false" ]; then
+    POLICY="unified"
+fi
+
 OUTDIR="$PROJECT_DIR/outputs/$TAG"
 if [ -d "$OUTDIR" ] && [ -f "$OUTDIR/metrics.jsonl" ]; then
     echo "[ERROR] Output directory $OUTDIR already exists with data. Use a different --tag."