feat: integrate FontSmoothing from gpucontext v0.22.0

[kolkov]

Summary

gpucontext v0.22.0 added PlatformProvider.FontSmoothing() — OS text anti-aliasing mode selection:

• FontSmoothingNone — aliased/bitmap text
• FontSmoothingGrayscale — grayscale AA
• FontSmoothingSubpixel — LCD subpixel AA (ClearType)

This is distinct from SubpixelLayout (pixel arrangement). FontSmoothing tells HOW to anti-alias.

Current State

• Interface compliance: mock updated in app_test.go
• SubpixelLayout already wired via ggcanvas
• FontSmoothing not yet wired to text rendering pipeline

Tasks

• Wire FontSmoothing() from PlatformProvider to gg text rendering
• Verify ggcanvas reads it during init (may already do so)
• Optionally expose in widget.Context for custom text widgets

References

• gpucontext v0.22.0: PlatformProvider.FontSmoothing()
• Detection: Windows SPI, macOS defaults, Linux Xft.antialias + fontconfig