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/docs/benchmarking/README.md

@@ -0,0 +1,7 @@
+# Benchmark Suites
+
+vLLM provides comprehensive benchmarking tools for performance testing and evaluation:
+
+- **[Benchmark CLI](./cli.md)**: `vllm bench` CLI tools and specialized benchmark scripts for interactive performance testing.
+- **[Parameter Sweeps](./sweeps.md)**: Automate `vllm bench` runs for multiple configurations, useful for [optimization and tuning](../configuration/optimization.md).
+- **[Performance Dashboard](./dashboard.md)**: Automated CI that publishes benchmarks on each commit.