Root cause: The refactored code was hardcoding torch.float16 for CUDA,
ignoring the --image-precision bf16 CLI argument. The Z-Image-Turbo model
requires bfloat16 precision - using float16 causes NaN values in the
image processor, resulting in all-black images.

Also restored the original model loading logic with:
- GGUF model detection (skip diffusers for GGUF)
- OOM retry with progressive memory optimization
- use_safetensors=True
- Sequential CPU offload support