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/chatbox.md

@@ -0,0 +1,36 @@
+# Chatbox
+
+[Chatbox](https://github.com/chatboxai/chatbox) is a desktop client for LLMs, available on Windows, Mac, Linux.
+
+It allows you to deploy a large language model (LLM) server with vLLM as the backend, which exposes OpenAI-compatible endpoints.
+
+## Prerequisites
+
+Set up the vLLM environment:
+
+```bash
+pip install vllm
+```
+
+## Deploy
+
+1. Start the vLLM server with the supported chat completion model, e.g.
+
+    ```bash
+    vllm serve qwen/Qwen1.5-0.5B-Chat
+    ```
+
+1. Download and install [Chatbox desktop](https://chatboxai.app/en#download).
+
+1. On the bottom left of settings, Add Custom Provider
+    - API Mode: `OpenAI API Compatible`
+    - Name: vllm
+    - API Host: `http://{vllm server host}:{vllm server port}/v1`
+    - API Path: `/chat/completions`
+    - Model: `qwen/Qwen1.5-0.5B-Chat`
+
+    ![Chatbox settings screen](../../assets/deployment/chatbox-settings.png)
+
+1. Go to `Just chat`, and start to chat:
+
+    ![Chatbot chat screen](../../assets/deployment/chatbox-chat.png)