[Edge][WP3] Add CPU OmniPlatform (AR + generation stages on vLLM's CPU backend)

* vllm_omni/platforms/cpu/platform.py: CPUOmniPlatform(OmniPlatform, CpuPlatform)
  selected when VLLM_TARGET_DEVICE=cpu or a +cpu vLLM wheel is installed
  (builtin plugin "cpu"; explicit CPU target short-circuits accelerator probes
  like vLLM does). NUMA-based memory queries, one logical device, no graph
  wrapper (sentinel type for isinstance checks), inductor opt-in via
  VLLM_OMNI_CPU_INDUCTOR=1, diffusion stages unsupported.
* vllm_omni/platforms/cpu/worker: CPUARWorker / CPUGenerationWorker compose
  vLLM's CPUWorker with the Omni mixin (XPU pattern); CPUARModelRunner /
  CPUGenerationModelRunner compose the Omni runners with CPUModelRunner
  (no graphs, no async omni output; sleep/wake return error ACKs).
* patch.py skips the CuMem patch on CPU-only torch; Qwen3-TTS speaker encoder
  and prompt-embeds dtype follow the model dtype (float32/float16 on CPU).
* vllm_omni/deploy/qwen3_tts.yaml: `platforms: cpu:` overlay (eager, no async
  scheduling, absolute kv_cache_memory_bytes, disjoint OMP thread ranges).
* requirements/cpu.txt (+psutil, intel-openmp), docs/getting_started/installation/cpu.md.

Measured (this x86 Xeon 8480C host, vllm 0.28.0+cpu, bf16, 12+4 threads):
Qwen3-TTS 0.6B CustomVoice end-to-end on CPU, 12 prompts: TTFA p50 292 ms,
RTF ~1.7 (25-frame chunk ~4.6 s), init 185 s. Result JSON + init timeline
committed under benchmarks/tts/edge_results/.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/getting_started/installation/cpu.md

@@ -0,0 +1,56 @@
+# CPU
+
+vLLM-Omni can run **autoregressive and generation stages** (for example the
+Qwen3-TTS talker → code2wav pipeline) on vLLM's CPU backend. Diffusion stages
+are not supported on CPU.
+
+## Requirements
+
+- x86-64 with AVX-512 (bf16 needs `avx512_bf16` or AMX) or aarch64 with NEON
+  (`fp16`/`bf16` extensions recommended). See the vLLM CPU installation page
+  for the supported ISA matrix.
+- A vLLM **CPU** build (`+cpu` wheel). The CUDA wheel does not contain the
+  CPU kernels.
+- Python 3.10–3.13.
+
+## Install
+
+```bash
+uv venv --python 3.12 && source .venv/bin/activate
+# vLLM CPU wheel (x86-64 example; see vLLM docs for aarch64 and per-commit wheels)
+uv pip install --torch-backend cpu \
+  https://github.com/vllm-project/vllm/releases/download/v0.28.0/vllm-0.28.0+cpu-cp38-abi3-manylinux_2_34_x86_64.whl
+# vLLM-Omni without re-resolving torch, then its CPU requirements
+VLLM_TARGET_DEVICE=cpu uv pip install --torch-backend cpu --no-deps -e .
+VLLM_TARGET_DEVICE=cpu uv pip install --torch-backend cpu -r requirements/cpu.txt
+```
+
+## Run
+
+```bash
+export VLLM_TARGET_DEVICE=cpu                    # selects the CPU OmniPlatform
+export LD_PRELOAD=$(python -c "import glob,sys; print(glob.glob(sys.prefix+'/lib/libiomp5.so')[0])")
+# one utterance, Qwen3-TTS 0.6B, CPU overlay from vllm_omni/deploy/qwen3_tts.yaml (platforms: cpu:)
+python examples/offline_inference/text_to_speech/qwen3_tts/end2end.py --query-type CustomVoice
+# or let the hardware probe size everything (threads, KV budget, dtype, chunking):
+python benchmarks/tts/stream_latency_bench.py --model Qwen/Qwen3-TTS-12Hz-0.6B-CustomVoice \
+  --query-type CustomVoice --deploy-profile auto
+```
+
+What the CPU platform changes (`vllm_omni/platforms/cpu/platform.py`):
+
+| Item | CPU behaviour |
+|---|---|
+| Workers | `CPUARWorker` / `CPUGenerationWorker` compose vLLM's `CPUWorker` with the Omni runners |
+| Scheduling | `async_scheduling=False` (forced by vLLM's `CpuPlatform`), no CUDA graphs |
+| Memory | `gpu_memory_utilization` is a NUMA-node memory fraction; prefer `kv_cache_memory_bytes` |
+| Threads | per-stage `VLLM_CPU_OMP_THREADS_BIND` ranges (`platforms: cpu:` overlay or `deploy_profile="auto"`) |
+| Compile | code-predictor `torch.compile` off by default; `VLLM_OMNI_CPU_INDUCTOR=1` enables it |
+| dtype | bf16 with `avx512_bf16`/AMX or ARM `bf16`, otherwise `float32` (`deploy_profile="auto"`) |
+| Sleep mode | not supported (returns an error ACK) |
+
+## Tests
+
+```bash
+VLLM_TARGET_DEVICE=cpu pytest tests/platforms/test_cpu_platform.py tests/worker/test_cpu_ar_model_runner.py -m "core_model and cpu"
+```