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/benchmarks/README.md

@@ -0,0 +1,20 @@
+# Benchmarks
+
+This directory used to contain vLLM's benchmark scripts and utilities for performance testing and evaluation.
+
+## Contents
+
+- **Serving benchmarks**: Scripts for testing online inference performance (latency, throughput)
+- **Throughput benchmarks**: Scripts for testing offline batch inference performance
+- **Specialized benchmarks**: Tools for testing specific features like structured output, prefix caching, long document QA, request prioritization, and multi-modal inference
+- **Dataset utilities**: Framework for loading and sampling from various benchmark datasets (ShareGPT, HuggingFace datasets, synthetic data, etc.)
+
+## Usage
+
+For detailed usage instructions, examples, and dataset information, see the [Benchmark CLI documentation](https://docs.vllm.ai/en/latest/benchmarking/cli/#benchmark-cli).
+
+For full CLI reference see:
+
+- <https://docs.vllm.ai/en/latest/cli/bench/latency.html>
+- <https://docs.vllm.ai/en/latest/cli/bench/serve.html>
+- <https://docs.vllm.ai/en/latest/cli/bench/throughput.html>