Add a code-driven HUD that appears in paint mode and binds live to the
active MeshPainter: four channel toggles, HSV pickers for albedo and
emission, sliders for metallic/smoothness/occlusion, emission intensity
and brush size. The HUD self-assembles its own overlay canvas and event
system at runtime.
Add a ModeChanged event to GameModeManager so the HUD can follow the
active mode, and guard MeshPainter against painting through the UI.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
