AMD GPU (ROCm/HIP) and especially Strix Halo discussion group

# Feature Request: AMD GPU (ROCm) Backend Support

## Summary
Add support for AMD GPUs via ROCm/HIP, similar to existing Metal backend for Apple Silicon.

## Hardware Tested
- AMD Ryzen AI Max+ 395 (16-core/32-thread APU)
- AMD Radeon 8060S Graphics (integrated GPU, 64GB VRAM)
- Ubuntu 26.04 LTS
- ROCm 6.x installed and working (verified with rocminfo)

## Current Situation
- ds4 compiles on Linux with \`-DDS4_NO_METAL\` flag
- CPU backend works but is extremely slow for 284B parameter model
- llama.cpp with HIP works on same hardware (182 t/s on small model)

## Expected Behavior
- Run ds4 with \`--backend cpu\` (or new \`--backend gpu\`) on AMD GPUs
- Full model support (DeepSeek V4 Flash IQ2/Q4)
- KV cache persistence support

## Implementation Suggestion
HIP/ROCm API is similar to Metal. The main components needed:
1. \`ds4_hip.h\` / \`ds4_hip.c\` - HIP tensor operations (analogous to ds4_metal.h/m)
2. Convert Metal shaders to HIP/compute kernels
3. Add backend selection in ds4_engine_options

## Additional Notes
- GPU detected as gfx1151, 64GB VRAM available
- ROCm stack is stable on this hardware
- Happy to test and provide feedback