smbc: SDL2 frontend — the game is playable

Closed #5 in 141dbf3.

src/host/ now holds the SDL2 frontend: video (256×240 texture, integer scale, nearest sampling), input (keyboard + SDL_GameController onto the joypad bitmap), and the entry point with fixed-step pacing at the NTSC frame rate — the deterministic core advances one frame per period regardless of render rate. The audio device opens with the pull contract #35's APU synthesis will feed; silent until then, and a failed open degrades to silent play rather than aborting.

The core stays headless: tests link no SDL, and --headless N keeps a device-free run for CI. Verified on macOS — window opens straight into the title screen with the attract demo running.