Remove FrameExecutor indirection by inlining its logic into RuntimeHostLoop. Add set_video_mode() for NTSC/PAL switching, set_joypad2_buttons() for player 2 input, and fix mapper scanline IRQ test timing.
Full NES emulation: CPU, PPU, APU, 47 mappers, iNES/NES 2.0 parsing. GTK4 desktop client with HeaderBar, pixel-perfect Cairo rendering, drag-and-drop ROM loading, and keyboard shortcuts. 187 tests covering core emulation, mappers, and runtime.
