- Add hover preview (MeshPaintPreview.shader) with a Preview Mode enum
(Off / Flat Decal / On Surface) sharing the paint placement path so
size, mask, alignment and rotation match the actual stroke.
- Preview Opacity now ranges up to 4.0 so faint/grey masks can be shown
more strongly (saturates to full opacity before the hardware blend).
- Add Paint On Move Only: stamp once on mouse-down, then only while the
cursor moves (GPU and CPU paths alike).
- Remove brushOpacity; brushColor.a is now the global opacity multiplier
over the mask in both backends (the alpha channel was previously unused
by the brush shader).
