fix: correct MiniMax H3 audio Euler steps (#1908)

src/model/diffusion/model.hpp

@@ -108,6 +108,8 @@ struct MiniMaxH3DiffusionExtra {
     int audio_length                                              = 0;
     float video_sigma_shift                                       = 12.f;
     float audio_sigma_shift                                       = 3.f;
+    // Negative when the outer sampler is not a single-evaluation Euler step.
+    float next_video_sigma = -1.f;
 };
 
 struct MiniT2IDiffusionExtra {