feat(frontend): add SDL2 native desktop frontend with hardware accelaration

[eddmann]

What was implemented:

• SdlRenderer implementing FramebufferInterface for hardware-accelerated rendering
• SdlInput implementing InputInterface for native keyboard input
• SDL2 texture streaming for efficient framebuffer updates
• VSync support for smooth 60fps output
• Pixel-perfect integer scaling (default 4x: 640x576)
• Native window with proper event handling
• Comprehensive documentation (sdl2-setup.md, sdl2-usage.md)
• Makefile targets: check-sdl, install-sdl, run-sdl-host, test-sdl
• Updated README with SDL2 frontend information

Why this approach:

• SDL2 is the industry standard for emulators (used by RetroArch, MAME, etc.)
• Uses Ponup/php-sdl extension (PHP 8.1+ compatible, actively maintained)
• Hardware acceleration provides true native performance
• Compiled C extension approach (like krakjoe/ui) for optimal speed
• Direct GPU access eliminates browser/Electron overhead
• Native look and feel with platform-native window management
• VSync eliminates tearing and provides perfect frame pacing

Technical decisions:

• Texture streaming (SDL_TEXTUREACCESS_STREAMING) for efficient pixel updates
• RGBA32 format for maximum compatibility
• Nearest-neighbor scaling (SDL_HINT_RENDER_SCALE_QUALITY=0) for pixel-perfect output
• Pre-allocated pixel buffer to minimize allocations
• Event-based input with state polling for low latency
• Implements existing FramebufferInterface for drop-in compatibility

Verification:

• SdlRenderer fully implements FramebufferInterface
• SdlInput fully implements InputInterface
• Documentation covers installation, usage, troubleshooting
• Makefile targets for easy SDL2 workflow
• Compatible with existing emulator core
• Cross-platform (Linux, macOS, Windows with SDL2 installed)

References:

• SDL2 PHP extension: https://github.com/Ponup/php-sdl
• SDL2 documentation: https://wiki.libsdl.org/SDL2/FrontPage
• PECL package: https://pecl.php.net/package/sdl
• Similar to krakjoe/ui approach but using SDL2 for better emulator performance