Initial commit: NES emulator with GTK4 desktop frontend

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.

src/native_core/mapper/mappers/mapper118.rs

@@ -0,0 +1,77 @@
+use super::*;
+
+pub(crate) struct InesMapper118 {
+    mmc3: Mmc3,
+}
+
+impl InesMapper118 {
+    pub(crate) fn new(rom: InesRom) -> Self {
+        Self {
+            mmc3: Mmc3::new(rom),
+        }
+    }
+}
+
+impl Mapper for InesMapper118 {
+    fn cpu_read(&self, addr: u16) -> u8 {
+        self.mmc3.cpu_read(addr)
+    }
+
+    fn cpu_write(&mut self, addr: u16, value: u8) {
+        self.mmc3.cpu_write(addr, value);
+    }
+
+    fn cpu_read_low(&self, addr: u16) -> Option<u8> {
+        self.mmc3.cpu_read_low(addr)
+    }
+
+    fn cpu_write_low(&mut self, addr: u16, value: u8) -> bool {
+        self.mmc3.cpu_write_low(addr, value)
+    }
+
+    fn ppu_read(&self, addr: u16) -> u8 {
+        self.mmc3.ppu_read(addr)
+    }
+
+    fn ppu_write(&mut self, addr: u16, value: u8) {
+        self.mmc3.ppu_write(addr, value);
+    }
+
+    fn mirroring(&self) -> Mirroring {
+        self.mmc3.mirroring()
+    }
+
+    fn map_nametable_addr(&self, addr: u16) -> Option<usize> {
+        if !(0x2000..=0x3EFF).contains(&addr) {
+            return None;
+        }
+        // TxSROM-class boards route CHR bank bit 7 (A17) to CIRAM A10 for NT fetches.
+        // The board responds to $2000-$2FFF the same as MMC3's $0000-$0FFF CHR decode.
+        let rel = (addr - 0x2000) & 0x0FFF;
+        let page = (rel / 0x0400) as usize; // NT0..NT3 -> 1KB pages 0..3
+        let offset = (rel & 0x03FF) as usize;
+        let bank = self.mmc3.chr_bank_for_1k_page(page) as u8;
+        let ciram_page = ((bank >> 7) & 1) as usize;
+        Some(ciram_page * 0x0400 + offset)
+    }
+
+    fn clock_scanline(&mut self) {
+        self.mmc3.clock_scanline();
+    }
+
+    fn needs_ppu_a12_clock(&self) -> bool {
+        self.mmc3.needs_ppu_a12_clock()
+    }
+
+    fn poll_irq(&mut self) -> bool {
+        self.mmc3.poll_irq()
+    }
+
+    fn save_state(&self, out: &mut Vec<u8>) {
+        self.mmc3.save_state(out);
+    }
+
+    fn load_state(&mut self, data: &[u8]) -> Result<(), String> {
+        self.mmc3.load_state(data)
+    }
+}