[Windows] RTX 5070 Ti (Blackwell) - cuInit error 100 fix guide

### Environment

- **GPU**: NVIDIA GeForce RTX 5070 Ti Laptop GPU (Blackwell, compute capability 12.0)
- **Driver**: 595.79 (CUDA 13.2)
- **OS**: Windows 11
- **PyTorch**: 2.11.0+cu130

### Problem

After installing ComfyUI on an RTX 5070 Ti laptop, CUDA was not available:
- torch.cuda.is_available() returned False
- cuInit() returned error code 100
- nvidia-smi worked fine, but no CUDA program could access the GPU

### Root Causes and Fixes

1. **Windows GPU Preference**: Hybrid graphics laptop routes Python to Intel iGPU by default. Fix: registry key GpuPreference=2 for python.exe
2. **PATH DLL conflict**: CUDA Toolkit cudart64_12.dll conflicts with PyTorch cu130 cudart64_13.dll. Fix: prioritize torch/lib in PATH
3. **System-level CUDA_VISIBLE_DEVICES=-1**: Must force-override to 0
4. **Blackwell architecture**: Needs TORCH_CUDA_ARCH_LIST=12.0

Full fix details and code available. RTX 5070 Ti/5080/5090 laptop users will hit this same issue.

Happy to contribute a PR with updated troubleshooting docs if helpful.