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/deployment/frameworks/open-webui.md

@@ -0,0 +1,42 @@
+# Open WebUI
+
+[Open WebUI](https://github.com/open-webui/open-webui) is an extensible, feature-rich,
+and user-friendly self-hosted AI platform designed to operate entirely offline.
+It supports various LLM runners like Ollama and OpenAI-compatible APIs,
+with built-in RAG capabilities, making it a powerful AI deployment solution.
+
+To get started with Open WebUI using vLLM, follow these steps:
+
+1. Install the [Docker](https://docs.docker.com/engine/install/).
+
+2. Start the vLLM server with a supported chat completion model:
+
+    ```console
+    vllm serve Qwen/Qwen3-0.6B-Chat
+    ```
+
+    !!! note
+        When starting the vLLM server, be sure to specify the host and port using the `--host` and `--port` flags.
+        For example:
+
+        ```console
+        vllm serve <model> --host 0.0.0.0 --port 8000
+        ```
+
+3. Start the Open WebUI Docker container:
+
+    ```console
+    docker run -d \
+        --name open-webui \
+        -p 3000:8080 \
+        -v open-webui:/app/backend/data \
+        -e OPENAI_API_BASE_URL=http://0.0.0.0:8000/v1 \
+        --restart always \
+        ghcr.io/open-webui/open-webui:main
+    ```
+
+4. Open it in the browser: <http://open-webui-host:3000/>
+
+    At the top of the page, you should see the model `Qwen/Qwen3-0.6B-Chat`.
+
+    ![Web portal of model Qwen/Qwen3-0.6B-Chat](../../assets/deployment/open_webui.png)