MB5 driver updates: PD-proxy + snapshot instrument + launcher tweaks

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

microbench/fresh_setup/mb5_run.sh

@@ -17,7 +17,8 @@
 set -eo pipefail
 
 FRESH_ROOT="/home/admin/cpfs/wjh/agentic-kv-fresh"
-VENV="${FRESH_ROOT}/.venv"
+# MB5_VENV lets a second host use an isolated venv clone (see mb5_launch.sh).
+VENV="${MB5_VENV:-${FRESH_ROOT}/.venv}"
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 LAUNCH="${SCRIPT_DIR}/mb5_launch.sh"
 REPLAYER_DIR="${FRESH_ROOT}/replayer"