- Implement unified Capturer trait with platform-specific stubs: DXGI (Win), CGDisplay (macOS), PipeWire (Linux) - Implement SoftwareEncoder using ffmpeg-next (libx264) with RGBA→YUV420P conversion, low-latency preset - Add create_capturer() factory function for platform auto-detection - Register capture and encoder Tauri commands (start_capture, init_encoder, get_display_resolution)