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/examples/online_serving/disaggregated_serving/README.md

@@ -0,0 +1,9 @@
+# Disaggregated Serving
+
+This example contains scripts that demonstrate the disaggregated serving features of vLLM.
+
+## Files
+
+- `disagg_proxy_demo.py` - Demonstrates XpYd (X prefill instances, Y decode instances).
+- `kv_events.sh` - Demonstrates KV cache event publishing.
+- `mooncake_connector` - A proxy demo for MooncakeConnector.