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/community/contact_us.md

@@ -0,0 +1,3 @@
+# Contact Us
+
+--8<-- "README.md:contact-us"

third_party/vllm/docs/community/meetups.md

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+# Meetups
+
+We host regular meetups around the world. We will share the project updates from the vLLM team and have guest speakers from the industry to share their experience and insights.
+
+Please visit [vllm.ai/events](https://vllm.ai/events) to learn more.

third_party/vllm/docs/community/sponsors.md

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+# Sponsors
+
+vLLM is a community project. Our compute resources for development and testing are supported by the following organizations. Thank you for your support!
+
+Please visit [vllm.ai/#sponsors](https://vllm.ai/#sponsors) to learn more.