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/requirements/tpu.txt

@@ -0,0 +1,14 @@
+# Common dependencies
+-r common.txt
+
+# Dependencies for TPU
+cmake>=3.26.1
+packaging>=24.2
+setuptools-scm>=8
+wheel
+jinja2>=3.1.6
+ray[default]
+ray[data]
+setuptools==78.1.0
+nixl==0.3.0
+tpu-inference==0.12.0