feat: use backend-native FP8 matmul when supported

[leejet]

Summary

• Detect whether the selected backend supports FP8 mul_mat.
• Preserve E4M3/E5M2 weights for native FP8 execution on capable backends.
• Fall back to BF16 weight conversion when native FP8 matmul is unavailable.

Related Issue / Discussion

N/A

Additional Information

.\bin\Release\sd-cli.exe --diffusion-model ..\models\diffusion_models\krea2_turbo_fp8_scaled.safetensors --llm ..\models\text_encoders\Qwen3-VL-4B-Instruct-Q4_K_M.gguf --vae ..\models\vae\wan_2.1_vae.safetensors -p "a lovely cat holding a sign says 'krea2.cpp'" --steps 8 --cfg-scale 1  --diffusion-fa -v --offload-to-cpu

Before: 2.52it/s
After: 5.35it/s

Test on RTX4090.

Checklist

• I have read and confirmed this PR follows the contribution guidelines.