Connector tax Phase A: build_connector_meta is 1.4ms/step (the tax source)

Per-step timing from engine_step.jsonl definitively resolves H3:
  plain:            53 μs/step (p50)
  noop_connector:   69 μs/step (+16 μs = negligible framework cost)
  mooncake_producer: 1461 μs/step (build_connector_meta = 1386 μs)
  mooncake_both:    1452 μs/step (same as producer)

The substrate tax is NOT in the v1 framework — it's specifically in
Mooncake's build_connector_meta() which walks set(cache.keys()) every
scheduler step (O(|cache|) per step, E2 audit §6.5).

Accumulated per-request tax: 256 decode steps × 1.4ms = 358ms.
Observed TTFT tax at rate=1.0: plain 378ms vs mooncake_both 422ms (+12%).
At rate=2.0 (near saturation): +29%, approaching trace-replay's +45%.

Also fixes kill_vllm() to properly kill EngineCore subprocesses.

microbench/connector_tax/run_all.sh

@@ -58,22 +58,29 @@ manifest() {
 
 kill_vllm() {
     local pidfile="$1"
+    # Kill the API server PID
     if [[ -f "$pidfile" ]]; then
         local pid; pid=$(cat "$pidfile")
         if [[ -n "$pid" ]]; then
             kill -9 "$pid" 2>/dev/null || true
         fi
     fi
-    pkill -f "port $PORT" 2>/dev/null || true
+    # Also kill any vLLM/EngineCore processes on this port or GPU
+    pkill -9 -f "port $PORT" 2>/dev/null || true
+    pkill -9 -f "VLLM::EngineCore" 2>/dev/null || true
+    pkill -9 -f "vllm.entrypoints" 2>/dev/null || true
     sleep 5
-    # wait for GPU release
+    # Wait for GPU memory release (up to 60s)
     for _ in $(seq 1 30); do
-        used=$(nvidia-smi --query-gpu=memory.used --format=csv,noheader,nounits | head -n $((GPU_ID + 1)) | tail -1)
-        if [[ "$used" -lt 1000 ]]; then
-            break
+        used=$(nvidia-smi --query-gpu=memory.used --format=csv,noheader,nounits -i "$GPU_ID" 2>/dev/null | tr -d ' ')
+        if [[ -n "$used" && "$used" -lt 1000 ]]; then
+            return 0
         fi
+        # Try killing any remaining GPU holders
+        fuser -k "/dev/nvidia${GPU_ID}" 2>/dev/null || true
         sleep 2
     done
+    echo "WARNING: GPU $GPU_ID still not free after 60s" >&2
 }
 
 run_phase_a() {