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/mkdocs/overrides/partials/toc-item.html

@@ -0,0 +1,21 @@
+<!-- Enables the use of toc_depth in document frontmatter https://github.com/squidfunk/mkdocs-material/issues/4827#issuecomment-1869812019 -->
+<li class="md-nav__item">
+    <a href="{{ toc_item.url }}" class="md-nav__link">
+      <span class="md-ellipsis">
+        {{ toc_item.title }}
+      </span>
+    </a>
+  
+    <!-- Table of contents list -->
+    {% if toc_item.children %}
+      <nav class="md-nav" aria-label="{{ toc_item.title | striptags }}">
+        <ul class="md-nav__list">
+          {% for toc_item in toc_item.children %}
+          {% if not page.meta.toc_depth or toc_item.level <= page.meta.toc_depth %}
+            {% include "partials/toc-item.html" %}
+          {% endif %}
+          {% endfor %}
+        </ul>
+      </nav>
+    {% endif %}
+  </li>