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/csrc/quantization/cutlass_w4a8/w4a8_utils.cuh

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <cstddef>
+#include "cutlass/numeric_types.h"
+
+namespace vllm::cutlass_w4a8_utils {
+
+bool unified_encode_int4b(cutlass::int4b_t const* in, cutlass::int4b_t* out,
+                          size_t num_int4_elems);
+
+}  // namespace vllm::cutlass_w4a8_utils