Add sampling gradient scaling based on surface area normalization

[akenmorris]

• Optimization: Image spacing greatly affects surface sampling #1227

• Add optional sampling gradient scaling that normalizes based on surface area, balancing optimization across shapes of different sizes
• Expose surface area from MeshDomain for scale factor computation
• Add Studio UI controls for sampling scale configuration
• Fix grooming crash on malformed meshes (no cells or non-triangular geometry)