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/design/optimization_levels.md

@@ -0,0 +1,86 @@
+# Optimization Levels
+
+## Overview
+
+vLLM provides 4 optimization levels (`-O0`, `-O1`, `-O2`, `-O3`) that allow users to trade off startup time for performance:
+
+- `-O0`: No optimization. Fastest startup time, but lowest performance.
+- `-O1`: Fast optimization. Simple compilation and fast fusions, and PIECEWISE cudagraphs.
+- `-O2`: Default optimization. Additional compilation ranges, additional fusions, FULL_AND_PIECEWISE cudagraphs.
+- `-O3`: Aggressive optimization. Currently equal to `-O2`, but may include additional time-consuming or experimental optimizations in the future.
+
+All optimization level defaults can be achieved by manually setting the underlying flags.
+User-set flags take precedence over optimization level defaults.
+
+## Level Summaries and Usage Examples
+
+```bash
+# CLI usage
+python -m vllm.entrypoints.api_server --model RedHatAI/Llama-3.2-1B-FP8 -O1
+
+# Python API usage
+from vllm.entrypoints.llm import LLM
+
+llm = LLM(
+    model="RedHatAI/Llama-3.2-1B-FP8",
+    optimization_level=2 # equivalent to -O2
+)
+```
+
+### `-O0`: No Optimization
+
+Startup as fast as possible - no autotuning, no compilation, and no cudagraphs.
+This level is good for initial phases of development and debugging.
+
+Settings:
+
+- `-cc.cudagraph_mode=NONE`
+- `-cc.mode=NONE` (also resulting in `-cc.custom_ops=["none"]`)
+- `-cc.pass_config.fuse_...=False` (all fusions disabled)
+- `--kernel-config.enable_flashinfer_autotune=False`
+
+### `-O1`: Fast Optimization
+
+Prioritize fast startup, but still enable basic optimizations like compilation and cudagraphs.
+This level is a good balance for most development scenarios where you want faster startup but
+still make sure your code does not break cudagraphs or compilation.
+
+Settings:
+
+- `-cc.cudagraph_mode=PIECEWISE`
+- `-cc.mode=VLLM_COMPILE`
+- `--kernel-config.enable_flashinfer_autotune=True`
+
+Fusions:
+
+- `-cc.pass_config.fuse_norm_quant=True`*
+- `-cc.pass_config.fuse_act_quant=True`*
+- `-cc.pass_config.fuse_act_padding=True`†
+- `-cc.pass_config.fuse_rope_kvcache=True`† (will be moved to O2)
+
+\* These fusions are only enabled when either op is using a custom kernel, otherwise Inductor fusion is better.</br>
+† These fusions are ROCm-only and require AITER.
+
+### `-O2`: Full Optimization (Default)
+
+Prioritize performance at the expense of additional startup time.
+This level is recommended for production workloads and is hence the default.
+Fusions in this level _may_ take longer due to additional compile ranges.
+
+Settings (on top of `-O1`):
+
+- `-cc.cudagraph_mode=FULL_AND_PIECEWISE`
+- `-cc.pass_config.fuse_allreduce_rms=True`
+
+### `-O3`: Aggressive Optimization
+
+This level is currently the same as `-O2`, but may include additional optimizations
+in the future that are more time-consuming or experimental.
+
+## Troubleshooting
+
+### Common Issues
+
+1. **Startup Time Too Long**: Use `-O0` or `-O1` for faster startup
+2. **Compilation Errors**: Use `debug_dump_path` for additional debugging information
+3. **Performance Issues**: Ensure using `-O2` for production