Split mesh painter into partials and add occlusion via combined mask map

- Split MeshPainter into partial classes (Setup/Backend/Gpu/Cpu) for maintainability
- Add ambient occlusion painting, packed with metallic/smoothness into one
  MSO mask map (R=metallic, G=occlusion, A=smoothness) bound to both
  _MetallicGlossMap and _OcclusionMap, kept independently paintable via
  per-channel ColorMask
- Gate painting from a single IsChannelActive source so AO-only strokes work
- Blend albedo/emission in linear space on CPU to match the GPU framebuffer
- Pre-compensate data-channel writes for sRGB->linear so GPU matches CPU
- Harden GPU shader fallback to reflect real backend state

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/MeshPainter.Backend.cs.meta

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