Fix LMetric routing: remove session affinity, align with OSDI'26 spec

LMetric was incorrectly sharing session-sticky logic with Linear policy.
Fixed to pure per-request routing: score = P_tokens × BS where
P = pending_prefill + (input - cache_hit), BS = num_requests.

Experiment result (200 req, fresh restart): Linear vs corrected LMetric
show <2% difference on all metrics — LMetric's cache-hit estimation
provides implicit soft affinity that preserves locality without explicit
session stickiness.

Also fix bench.sh missing cd (replayer module not found from non-project
cwd) and rewrite run_lmetric_ab.sh as thin wrapper around bench.sh to
eliminate duplicated launch/cleanup logic that broke under set -euo.

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

scripts/bench.sh

@@ -328,6 +328,7 @@ echo "  mode=$MODE  policy=$POLICY  requests=$REQUESTS  overload_factor=${OVERLO
 echo "  $(date)"
 echo "================================================================"
 
+cd "$PROJECT_DIR"
 cleanup_gpu
 launch_instances
 launch_proxy