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/tests/evals/gsm8k/README.md

@@ -0,0 +1,40 @@
+# GSM8K Accuracy Evaluation
+
+This directory contains a replacement for the lm-eval-harness GSM8K evaluation, using an isolated GSM8K script and vLLM server for better performance and control.
+
+## Usage
+
+### Run tests with pytest (like buildkite)
+
+```bash
+pytest -s -v tests/evals/gsm8k/test_gsm8k_correctness.py \
+    --config-list-file=configs/models-small.txt
+```
+
+### Run standalone evaluation script
+
+```bash
+# Start vLLM server first
+vllm serve Qwen/Qwen2.5-1.5B-Instruct --port 8000
+
+# Run evaluation
+python tests/evals/gsm8k/gsm8k_eval.py --port 8000
+```
+
+## Configuration Format
+
+Model configs in `configs/` directory use this YAML format:
+
+```yaml
+model_name: "Qwen/Qwen2.5-1.5B-Instruct"
+accuracy_threshold: 0.54  # Minimum expected accuracy
+num_questions: 1319       # Number of questions (default: full test set)
+num_fewshot: 5            # Few-shot examples from train set
+server_args: "--max-model-len 4096 --tensor-parallel-size 2"  # Server arguments
+env:                      # Environment variables (optional)
+  VLLM_USE_FLASHINFER_MOE_FP4: "1"
+```
+
+The `server_args` field accepts any arguments that can be passed to `vllm serve`.
+
+The `env` field accepts a dictionary of environment variables to set for the server process.