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/configuration/README.md

@@ -0,0 +1,9 @@
+# Configuration Options
+
+This section lists the most common options for running vLLM.
+
+There are three main levels of configuration, from highest priority to lowest priority:
+
+- [Request parameters](../serving/openai_compatible_server.md#completions-api) and [input arguments](../api/README.md#inference-parameters)
+- [Engine arguments](./engine_args.md)
+- [Environment variables](./env_vars.md)