perf(wan): Add mx.compile and fix first-frame artifacts

mlx_video/models/wan/vae22.py

@@ -81,7 +81,8 @@ class CausalConv3d(nn.Module):
             y = mx.conv_general(x_flat, w2d) + self.bias
             return y.reshape(B, T, y.shape[1], y.shape[2], -1)
 
-        # Causal temporal padding (left only)
+        # Causal temporal padding (left only) — zeros match the reference
+        # implementation and what the model was trained with.
         if self._causal_pad_t > 0:
             pad_t = mx.zeros((B, self._causal_pad_t, H, W, C))
             x = mx.concatenate([pad_t, x], axis=1)