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/test_support.rs

@@ -0,0 +1,114 @@
+use crate::native_core::cpu::{Cpu6502, CpuBus};
+use crate::native_core::ines::{InesHeader, InesRom, Mirroring};
+
+pub(crate) struct TestRamBus(pub [u8; 0x10000]);
+
+impl TestRamBus {
+    pub(crate) fn new() -> Self {
+        Self([0; 0x10000])
+    }
+
+    pub(crate) fn load(&mut self, start: u16, bytes: &[u8]) {
+        for (i, b) in bytes.iter().copied().enumerate() {
+            self.0[start as usize + i] = b;
+        }
+    }
+}
+
+impl CpuBus for TestRamBus {
+    fn read(&mut self, addr: u16) -> u8 {
+        self.0[addr as usize]
+    }
+
+    fn write(&mut self, addr: u16, value: u8) {
+        self.0[addr as usize] = value;
+    }
+}
+
+pub(crate) fn cpu_setup_with_reset(prog: &[u8]) -> (Cpu6502, TestRamBus) {
+    let mut bus = TestRamBus::new();
+    bus.load(0x8000, prog);
+    bus.0[0xFFFC] = 0x00;
+    bus.0[0xFFFD] = 0x80;
+    let mut cpu = Cpu6502::default();
+    cpu.reset(&mut bus);
+    (cpu, bus)
+}
+
+pub(crate) struct MapperRomBuilder {
+    mapper: u16,
+    submapper: u8,
+    prg_banks_16k: u16,
+    chr_banks_8k: u16,
+    mirroring: Mirroring,
+}
+
+impl MapperRomBuilder {
+    pub(crate) fn new(mapper: u16) -> Self {
+        Self {
+            mapper,
+            submapper: 0,
+            prg_banks_16k: 1,
+            chr_banks_8k: 1,
+            mirroring: Mirroring::Horizontal,
+        }
+    }
+
+    pub(crate) fn submapper(mut self, submapper: u8) -> Self {
+        self.submapper = submapper;
+        self
+    }
+
+    pub(crate) fn prg_banks_16k(mut self, banks: u16) -> Self {
+        self.prg_banks_16k = banks;
+        self
+    }
+
+    pub(crate) fn chr_banks_8k(mut self, banks: u16) -> Self {
+        self.chr_banks_8k = banks;
+        self
+    }
+
+    pub(crate) fn mirroring(mut self, mirroring: Mirroring) -> Self {
+        self.mirroring = mirroring;
+        self
+    }
+
+    pub(crate) fn build(self) -> InesRom {
+        let prg_rom_len = self.prg_banks_16k as usize * 16 * 1024;
+        let chr_len = if self.chr_banks_8k == 0 {
+            8 * 1024
+        } else {
+            self.chr_banks_8k as usize * 8 * 1024
+        };
+        let chr_data = if self.chr_banks_8k == 0 {
+            vec![0; chr_len]
+        } else {
+            (0..chr_len).map(|v| (v & 0xFF) as u8).collect()
+        };
+        InesRom {
+            header: InesHeader {
+                mapper: self.mapper,
+                submapper: self.submapper,
+                is_nes2: false,
+                prg_rom_banks_16k: self.prg_banks_16k,
+                chr_rom_banks_8k: self.chr_banks_8k,
+                has_trainer: false,
+                has_battery: false,
+                mirroring: self.mirroring,
+                prg_ram_shift: 0,
+                prg_nvram_shift: 0,
+                chr_ram_shift: 0,
+                chr_nvram_shift: 0,
+                cpu_ppu_timing_mode: 0,
+                vs_hardware_type: 0,
+                vs_ppu_type: 0,
+                misc_rom_count: 0,
+                default_expansion_device: 0,
+            },
+            prg_rom: (0..prg_rom_len).map(|v| (v & 0xFF) as u8).collect(),
+            chr_data,
+            chr_is_ram: self.chr_banks_8k == 0,
+        }
+    }
+}