docs: add GD module development plan

[fglock]

Summary

• Add development plan for implementing the Perl GD module on PerlOnJava
• Backend: java.awt.Graphics2D + BufferedImage (JDK built-in, zero new Maven dependencies)
• GD has no CLI tool and no pure-Perl fallback, so Java XS is the only viable approach
• Plan covers ~147 XS functions across 3 phases:
  • Phase 1 (Core MVP): Image creation, drawing primitives, color management, PNG/JPEG/GIF I/O (~35 functions)
  • Phase 2: Copy/merge, brushes, styles, full polygon support (~25 functions)
  • Phase 3: Filters, transforms, GIF animation, TIFF/WebP via TwelveMonkeys (~40 functions)
• Documents downstream consumers that would become usable: GD::Graph, GD::Barcode, GD::SecurityImage, PDF::API2, etc.
• Pure Perl files from CPAN (GD::Polygon, GD::Simple, GD::Polyline, GD::Group) can be reused as-is

Test plan

• Documentation only — no code changes
• Verified no other files modified

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