Add NES emulation support with bundled FCEUmm core

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.

app/src/main/cpp/libretro_frontend.h

@@ -9,6 +9,7 @@
 #include <string>
 #include <thread>
 #include <unordered_map>
+#include <vector>
 
 enum class DisplayMode : int {
     STRETCH = 0,
@@ -43,6 +44,8 @@ public:
 
     void setSystemDir(const char* dir);
     void setSaveDir(const char* dir);
+    void setControllerPortDevice(unsigned port, unsigned device);
+    void setCoreOption(const char* key, const char* value);
 
     bool isRunning() const { return running_.load(); }
 
@@ -114,6 +117,10 @@ private:
     std::string system_dir_;
     std::string save_dir_;
 
+    // ROM data (kept alive for cores with need_fullpath=false)
+    std::vector<uint8_t> rom_data_;
+    bool need_fullpath_ = true;
+
     // Frame timing
     double target_fps_ = 60.0;
     int frame_skip_ = 0;