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/engine_args.md

@@ -0,0 +1,22 @@
+---
+toc_depth: 3
+---
+
+# Engine Arguments
+
+Engine arguments control the behavior of the vLLM engine.
+
+- For [offline inference](../serving/offline_inference.md), they are part of the arguments to [LLM][vllm.LLM] class.
+- For [online serving](../serving/openai_compatible_server.md), they are part of the arguments to `vllm serve`.
+
+The engine argument classes, [EngineArgs][vllm.engine.arg_utils.EngineArgs] and [AsyncEngineArgs][vllm.engine.arg_utils.AsyncEngineArgs], are a combination of the configuration classes defined in [vllm.config][]. Therefore, if you are interested in developer documentation, we recommend looking at these configuration classes as they are the source of truth for types, defaults and docstrings.
+
+--8<-- "docs/cli/json_tip.inc.md"
+
+## `EngineArgs`
+
+--8<-- "docs/generated/argparse/engine_args.inc.md"
+
+## `AsyncEngineArgs`
+
+--8<-- "docs/generated/argparse/async_engine_args.inc.md"