docs: record Qwen MLX benchmark results

benchmarks/inference/results/backend_verification.json

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+{
+  "verified_at": "2026-08-31",
+  "host": "Apple M4 Max, arm64, macOS 15.6",
+  "ollama": {
+    "version": "0.32.14",
+    "mlx_version": "0.32.0-213-gadf21de",
+    "device": "gpu",
+    "metal_library": true,
+    "nvfp4_model": {
+      "name": "qwen3.8:27b-mlx",
+      "id": "5642e97495e1",
+      "format": "safetensors",
+      "quantization": "nvfp4",
+      "parameter_size": "27.8B",
+      "runner_log": "starting mlx runner subprocess"
+    },
+    "mxfp8_model": {
+      "name": "qwen3.8:27b-mxfp8",
+      "id": "464021588235",
+      "format": "safetensors",
+      "quantization": "mxfp8",
+      "parameter_size": "27.8B",
+      "runner_log": "starting mlx runner subprocess"
+    }
+  },
+  "vllm_metal": {
+    "vllm_version": "0.28.0+cpu",
+    "plugin_version": "0.4.0.dev20260831112704",
+    "mlx_version": "0.32.0",
+    "mlx_lm_version": "0.31.3",
+    "mlx_vlm_version": "0.6.4",
+    "platform_plugin": "metal",
+    "compute_log": "MLX device set to: Device(gpu, 0)",
+    "pytorch_device_log": "PyTorch device set to: mps",
+    "note": "The +cpu suffix identifies the portable vLLM core wheel; the Metal plugin supplies MLX GPU execution."
+  }
+}