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/maybe_skip_pr_build.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+# SPDX-License-Identifier: Apache-2.0
+# Skip PR builds unless the PR has the "documentation" or "ready" label.
+# Used by Read the Docs (see .readthedocs.yaml).
+
+if [[ "$READTHEDOCS_VERSION_TYPE" != "external" ]]; then
+  exit 0
+fi
+
+PR_URL="https://api.github.com/repos/vllm-project/vllm/pulls/${READTHEDOCS_VERSION}"
+CURL_ARGS=(-s -o /tmp/pr_response.json -w "%{http_code}")
+if [[ -n "$GITHUB_TOKEN" ]]; then
+  CURL_ARGS+=(-H "Authorization: token ${GITHUB_TOKEN}")
+fi
+HTTP_CODE=$(curl "${CURL_ARGS[@]}" "$PR_URL")
+
+if [[ "$HTTP_CODE" -ne 200 ]]; then
+  echo "GitHub API returned HTTP ${HTTP_CODE}, proceeding with build."
+elif grep -qE '"name": *"(documentation|ready)"' /tmp/pr_response.json; then
+  echo "Found required label, proceeding with build."
+else
+  echo "PR #${READTHEDOCS_VERSION} lacks 'documentation' or 'ready' label, cancelling build."
+  exit 1
+fi