audio.cpp 0.4 adds Higgs Audio v3 TTS and Fish Audio S2 Pro support

Release 0.4 adds Higgs Audio v3 TTS 4B (10x real time), Fish Audio S2 Pro, Voxtral Realtime ASR, OuteTTS and VieNeu-TTS-v3 in C++/GGML. GGUF loading is now first-class across the project, with Q8 speed and VRAM gains.