Initial commit: Unity mesh painter project

Unity 6000.3 (URP) project with live mesh painting in play mode.

- MeshPainter.cs: paint a SkinnedMesh with the mouse, seamless across
  UV islands via 3D-space painting, GPU and CPU backends (switchable),
  edge-padding/dilation against bilinear seam bleed
- FrameRateLimiter.cs: simple frame rate limiting component
- MeshPaintBrush / MeshPaintDilate shaders for the GPU paint path
- Unity-specific .gitignore and .gitattributes

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

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