Add z-image image generation prototype (#13659)

x/imagegen/cmd/engine/README.md

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+# MLX Engine
+
+Experimental MLX backend for running models on Apple Silicon and CUDA.
+
+## Build
+
+```bash
+go build -tags mlx -o engine ./x/imagegen/cmd/engine
+```
+
+## Text Generation
+
+```bash
+./engine -model /path/to/model -prompt "Hello" -max-tokens 100
+```
+
+Options:
+
+- `-temperature` - sampling temperature (default 0.7)
+- `-top-p` - nucleus sampling (default 0.9)
+- `-top-k` - top-k sampling (default 40)
+
+Supports: Llama, Gemma3, GPT-OSS
+
+## Image Generation
+
+```bash
+./engine -zimage -model /path/to/z-image -prompt "a cat" -output cat.png
+```
+
+Options:
+
+- `-width`, `-height` - image dimensions (default 1024x1024)
+- `-steps` - denoising steps (default 9)
+- `-seed` - random seed (default 42)