Add model card metadata

Point the q8dit rows at audio.cpp#464

Document the mixed-precision q8dit builds

Add XL SFT mixed-precision GGUF (q8_0 DiT, bf16 planner/text encoder/VAE)

Add XL Turbo mixed-precision GGUF (q8_0 DiT, bf16 planner/text encoder/VAE)

Add the XL SFT GGUF to the README

ACE-Step 1.5 XL SFT, bf16 GGUF for audio.cpp

Describe the ACE-Step 1.5 XL Turbo GGUF

ACE-Step 1.5 XL Turbo, bf16 GGUF for audio.cpp

initial commit