Add in-game paint HUD bound to the mesh painter

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>

Assets/Scripts/HsvColorPicker.cs.meta

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