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/tools/pre_commit/shellcheck.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+set -euo pipefail
+
+scversion="stable"
+
+if [ -d "shellcheck-${scversion}" ]; then
+    export PATH="$PATH:$(pwd)/shellcheck-${scversion}"
+fi
+
+if ! [ -x "$(command -v shellcheck)" ]; then
+    if [ "$(uname -s)" != "Linux" ] || [ "$(uname -m)" != "x86_64" ]; then
+        echo "Please install shellcheck: https://github.com/koalaman/shellcheck?tab=readme-ov-file#installing"
+        exit 1
+    fi
+
+    # automatic local install if linux x86_64
+    wget -qO- "https://github.com/koalaman/shellcheck/releases/download/${scversion?}/shellcheck-${scversion?}.linux.x86_64.tar.xz" | tar -xJv
+    export PATH="$PATH:$(pwd)/shellcheck-${scversion}"
+fi
+
+# TODO - fix warnings in .buildkite/scripts/hardware_ci/run-amd-test.sh
+find . -path ./.git -prune -o -name "*.sh" \
+  -not -path "./.buildkite/scripts/hardware_ci/run-amd-test.sh" -print0 | \
+  xargs -0 sh -c "for f in \"\$@\"; do git check-ignore -q \"\$f\" || shellcheck -s bash \"\$f\"; done" --