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/streamlit.md

@@ -0,0 +1,38 @@
+# Streamlit
+
+[Streamlit](https://github.com/streamlit/streamlit) lets you transform Python scripts into interactive web apps in minutes, instead of weeks. Build dashboards, generate reports, or create chat apps.
+
+It can be quickly integrated with vLLM as a backend API server, enabling powerful LLM inference via API calls.
+
+## Prerequisites
+
+Set up the vLLM environment by installing all required packages:
+
+```bash
+pip install vllm streamlit openai
+```
+
+## Deploy
+
+1. Start the vLLM server with a supported chat completion model, e.g.
+
+    ```bash
+    vllm serve Qwen/Qwen1.5-0.5B-Chat
+    ```
+
+1. Use the script: [examples/online_serving/streamlit_openai_chatbot_webserver.py](../../../examples/online_serving/streamlit_openai_chatbot_webserver.py)
+
+1. Start the streamlit web UI and start to chat:
+
+    ```bash
+    streamlit run streamlit_openai_chatbot_webserver.py
+
+    # or specify the VLLM_API_BASE or VLLM_API_KEY
+    VLLM_API_BASE="http://vllm-server-host:vllm-server-port/v1" \
+        streamlit run streamlit_openai_chatbot_webserver.py
+
+    # start with debug mode to view more details
+    streamlit run streamlit_openai_chatbot_webserver.py --logger.level=debug
+    ```
+
+    ![Chat with vLLM assistant in Streamlit](../../assets/deployment/streamlit-chat.png)