Implement NesCore with FCEUmm/Nestopia/Mesen libretro core support. Generalize native frontend to handle cores that require ROM data in memory (need_fullpath=false) and move PS1-specific config out of C++ into Ps1Core. Bundle both core .so files as APK assets with automatic extraction. Fix cover art fetching for GoodNES-named ROMs by expanding region codes (U)->(USA, Europe) to match LibRetro thumbnail naming.