MB5 proxy: session-affinity P routing (MB5_P_ROUTING=session)

The upstream mooncake_connector_proxy round-robins both P and D
selection. For agentic multi-turn sessions this destroys prefix-cache
reuse on the producer side — every turn of a session lands on a
different P, so the prefix-cache hit ratio collapses to 0 (observed in
the 6P+2D round-robin baseline) and every turn re-prefills from
scratch, piling extra load on the P pool.

Add an env-gated routing mode so the same proxy serves both arms of a
clean A/B:
  MB5_P_ROUTING=rr       round-robin (default, = upstream behavior)
  MB5_P_ROUTING=session  consistent md5 hash on X-Session-Id -> same
                         producer for all turns of a session

Decode side stays round-robin (load balance) in both modes — decode
KV is freshly transferred per turn, so D gains nothing from affinity
but everything from even load spreading.

mb5_launch.sh threads MB5_P_ROUTING through to the proxy and logs the
active mode. Default path is byte-for-byte the old behavior, so an
in-flight round-robin sweep is unaffected if this is redeployed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

microbench/fresh_setup/mb5_launch.sh

@@ -190,7 +190,9 @@ for port in "${all_ports[@]}"; do
 done
 
 if [ "${ROLES}" = "pd" ]; then
-    echo "[mb5] launching mooncake_connector_proxy on ${PROXY_PORT}"
+    P_ROUTING="${MB5_P_ROUTING:-rr}"
+    echo "[mb5] launching mooncake_connector_proxy on ${PROXY_PORT} (P routing=${P_ROUTING})"
+    MB5_P_ROUTING="${P_ROUTING}" \
     nohup python "${PROXY_SRC}" "${proxy_args[@]}" --port "${PROXY_PORT}" --host 0.0.0.0 \
         > "${LOGS_DIR}/proxy.log" 2>&1 &
     disown