Set PYTHONHASHSEED=42 for elastic mode to ensure consistent block hashes

Root cause confirmed: NONE_HASH = os.urandom(32) differs between
scheduler and bootstrap server even in the same process (init_none_hash
called separately by each import path). PYTHONHASHSEED makes it
deterministic: NONE_HASH = hash_fn(seed), same across all code paths.

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

scripts/bench.sh

@@ -133,6 +133,7 @@ launch_instances() {
         local logfile="$OUTDIR/vllm_inst_${i}.log"
 
         if [ "$MODE" = "elastic" ]; then
+            PYTHONHASHSEED=42 \
             VLLM_MOONCAKE_BOOTSTRAP_PORT=$((8998 + i)) \
             MASTER_PORT=$master \
             CUDA_VISIBLE_DEVICES=$i \