Make SDL3 the default runner frontend

[mstan]

Summary

• make SDL3 the default interactive runner frontend via NDS_SDL_BACKEND=SDL3
• keep explicit SDL2 compatibility and NDS_SDL_BACKEND=NONE headless builds
• port frontend/audio/controller/window helpers and compute GL host calls across SDL2/SDL3
• fix SDL3 bottom-screen touch by converting absolute mouse x/y from window coordinates to renderer logical coordinates while preserving relative xrel/yrel for aim

Validation

• configured and built SDL3 runner with compute enabled
• configured and built SDL2 fallback runner with compute enabled
• configured and built NDS_SDL_BACKEND=NONE headless runner with compute disabled
• built MPH title banks and title-aware SDL3 runner
• launched MPH through corrected direct/no-dump flags and verified active non-white framebuffers
• user manually confirmed the SDL3 recomp-ui launch path passes, including bottom-screen touch
• git diff --check

Notes

• MPH first-target integration is in Adopt SDL3 default for Prime Hunters MetroidPrimeHuntersRecomp#35 and should land with or after this framework PR.