plaster/smooth-stucco Unity mask/smoothness pack

Files:
- baseColor.png: import as color/albedo.
- normal.png: mark as Normal Map.
- maskMap.png: R=Metallic, G=AO, B=Detail Mask (black), A=Smoothness.
- metallicSmoothness.png: RGB=Metallic, A=Smoothness for Built-in/URP metallic workflows.
- height.png: optional displacement/parallax source.

Smoothness is generated as inverse roughness.
