Add vLLM v0.18.1 source tree with KV transfer abort fix

third_party/vllm/ now tracked in git for direct patch management.
Based on vLLM v0.18.1 release with one patch applied:

  vllm/v1/core/sched/scheduler.py:
    Replace fatal assert with graceful skip when KV transfer callback
    arrives for an already-aborted request during PD disaggregated serving.

Future vLLM modifications should be made directly in third_party/vllm/
and committed normally. The patches/ directory is kept as documentation
of what changed from upstream.

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

third_party/vllm/examples/online_serving/elastic_ep/bench.sh

@@ -0,0 +1,57 @@
+#!/bin/bash
+
+MODEL_NAME="deepseek-ai/DeepSeek-V2-Lite"
+LOCAL_MODEL_PATH="/models/models--deepseek-ai--DeepSeek-V2-Lite/snapshots/604d5664dddd88a0433dbae533b7fe9472482de0"
+HOST="localhost"
+PORT=8006
+NUM_PROMPTS=20
+REQUEST_RATE=5
+
+# Parse command line arguments
+while [[ $# -gt 0 ]]; do
+    case $1 in
+        --model)
+            MODEL_NAME="$2"
+            shift 2
+            ;;
+        --local-model)
+            MODEL_NAME=$LOCAL_MODEL_PATH
+            shift
+            ;;
+        --host)
+            HOST="$2"
+            shift 2
+            ;;
+        --port)
+            PORT="$2"
+            shift 2
+            ;;
+        --num-prompts)
+            NUM_PROMPTS="$2"
+            shift 2
+            ;;
+        --request-rate)
+            REQUEST_RATE="$2"
+            shift 2
+            ;;
+        -h|--help)
+            echo "Usage: $0 [OPTIONS]"
+            echo "Options:"
+            echo "  --model MODEL_NAME           Set model name or path (default: deepseek-ai/DeepSeek-V2-Lite)"
+            echo "  --local-model                Use local model path (convenience option)"
+            exit 0
+            ;;
+        *)
+            echo "Unknown option: $1"
+            echo "Use -h or --help for usage information"
+            exit 1
+            ;;
+    esac
+done
+
+vllm bench serve \
+    --model "$MODEL_NAME" \
+    --host "$HOST" \
+    --port "$PORT" \
+    --num-prompts "$NUM_PROMPTS" \
+    --request-rate "$REQUEST_RATE"