White horizontal plane artifact in generated GLB File

**Description**

When using SAM3D for single-image 3D reconstruction, the generated GLB mesh often includes an unwanted **flat horizontal plane** at the bottom or top of the object. This plane appears as a white/gray flat surface that is not part of the original object.

**Problem Details**

The artifact appears as:
- A flat horizontal mesh at the bottom of the reconstructed object (most common)
- Sometimes at the top of the object
- The plane has normals pointing up (Y-axis) or down
- It appears regardless of the input image content

**Questions**

- Is there a parameter or configuration to prevent this artifact?
- Is this related to the layout prediction or the Gaussian splatting to mesh conversion?
- Are there plans to address this in future releases?

**Related Issues**

#123 - Generate weird cross objects (similar geometric artifacts)
#128 - Improving geometry and back-view reconstruction

Thanks for the great work on SAM 3D, any guidance on this issue would be appreciated !