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/.github/workflows/macos-smoke-test.yml

@@ -0,0 +1,84 @@
+name: macOS Apple Silicon Smoke Test
+
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch:  # Manual trigger
+
+permissions:
+  contents: read
+
+jobs:
+  macos-m1-smoke-test:
+    runs-on: macos-latest
+    timeout-minutes: 30
+
+    steps:
+      - uses: actions/checkout@v6.0.1
+
+      - uses: astral-sh/setup-uv@v7
+        with:
+          enable-cache: true
+          cache-dependency-glob: |
+            requirements/**/*.txt
+            pyproject.toml
+          python-version: '3.12'
+
+      - name: Create virtual environment
+        run: |
+          uv venv
+          echo "$GITHUB_WORKSPACE/.venv/bin" >> "$GITHUB_PATH"
+
+      - name: Install dependencies and build vLLM
+        run: |
+          uv pip install -r requirements/cpu-build.txt --index-strategy unsafe-best-match
+          uv pip install -r requirements/cpu.txt --index-strategy unsafe-best-match
+          uv pip install -e . --no-build-isolation
+        env:
+          CMAKE_BUILD_PARALLEL_LEVEL: 4
+
+      - name: Verify installation
+        run: |
+          python -c "import vllm; print(f'vLLM version: {vllm.__version__}')"
+
+      - name: Smoke test vllm serve
+        run: |
+          # Start server in background
+          vllm serve Qwen/Qwen3-0.6B \
+            --max-model-len=2K \
+            --load-format=dummy \
+            --hf-overrides '{"num_hidden_layers": 2}' \
+            --enforce-eager \
+            --port 8000 &
+
+          SERVER_PID=$!
+
+          # Wait for server to start
+          for i in {1..30}; do
+            if curl -s http://localhost:8000/health > /dev/null; then
+              echo "Server started successfully"
+              break
+            fi
+            if [ "$i" -eq 30 ]; then
+              echo "Server failed to start"
+              kill "$SERVER_PID"
+              exit 1
+            fi
+            sleep 2
+          done
+
+          # Test health endpoint
+          curl -f http://localhost:8000/health
+
+          # Test completion
+          curl -f http://localhost:8000/v1/completions \
+            -H "Content-Type: application/json" \
+            -d '{
+              "model": "Qwen/Qwen3-0.6B",
+              "prompt": "Hello",
+              "max_tokens": 5
+            }'
+
+          # Cleanup
+          kill "$SERVER_PID"