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/.shellcheckrc

@@ -0,0 +1,9 @@
+# rules currently disabled:
+#
+#   SC1091 (info): Not following: <sourced file> was not specified as input (see shellcheck -x)
+#   SC2004 (style): $/${} is unnecessary on arithmetic variables.
+#   SC2129 (style): Consider using { cmd1; cmd2; } >> file instead of individual redirects.
+#   SC2155 (warning): Declare and assign separately to avoid masking return values.
+#   SC2164 (warning): Use 'cd ... || exit' or 'cd ... || return' in case cd fails.
+#
+disable=SC1091,SC2004,SC2129,SC2155,SC2164