Implement full color support (Game Boy Color)

[eddmann]

Description

Add complete Game Boy Color (CGB) support to the emulator, including color palette handling and rendering.

Tasks

• Implement CGB palette registers (BCPS/BCPD, OCPS/OCPD)
• Add background and sprite color palette RAM
• Update PPU to render in color mode
• Handle CGB-specific VRAM banking
• Support CGB color correction/gamma
• Add CGB detection and mode switching
• Test with CGB-only and dual-mode games

Acceptance Criteria

• CGB games render with correct colors
• Palette switching works correctly
• Backward compatibility with DMG games is maintained

References

• Pan Docs - CGB Registers
• Pan Docs - Palettes