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/png-lint.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+# Ensure that *.excalidraw.png files have the excalidraw metadata
+# embedded in them. This ensures they can be loaded back into
+# the tool and edited in the future.
+
+find . -iname '*.excalidraw.png' | while read -r file; do
+	if git check-ignore -q "$file"; then
+		continue
+	fi
+	if ! grep -q "excalidraw+json" "$file"; then
+		echo "$file was not exported from excalidraw with 'Embed Scene' enabled."
+		exit 1
+	fi
+done