Paint: Vertex Color painting tool (MVP)

[fernandotonon]

Summary

Add a vertex-color (RGBA) painting tool (polypaint-style) that paints directly on mesh vertices.

Acceptance criteria

• Brush paints vertex colors on hovered/dragged surface.
• Controls: radius, strength, falloff, color.
• Works with multi-select entities (paint active entity at minimum).
• Rendering toggle: view vertex colors in viewport.

Implementation notes

• Use raycast/picking to find impacted vertices around hit point.
• Update vertex buffers efficiently.