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/bus.rs

@@ -0,0 +1,93 @@
+use crate::native_core::{apu::Apu, cpu::CpuBus, mapper::Mapper, ppu::Ppu};
+
+const CPU_RAM_SIZE: usize = 0x0800;
+const PPU_DOTS_PER_SCANLINE: u32 = 341;
+const PPU_SCANLINES_PER_FRAME: u32 = 262;
+const PPU_DOTS_PER_FRAME: u32 = PPU_DOTS_PER_SCANLINE * PPU_SCANLINES_PER_FRAME;
+const PPU_VBLANK_START_SCANLINE: u32 = 241;
+const PPU_PRERENDER_SCANLINE: u32 = 261;
+
+pub struct NativeBus {
+    cpu_ram: [u8; CPU_RAM_SIZE],
+    ppu: Ppu,
+    apu: Apu,
+    cpu_open_bus: u8,
+    joypad_state: u8,
+    joypad_shift: u8,
+    joypad2_state: u8,
+    joypad2_shift: u8,
+    joypad_strobe: bool,
+    nmi_pending: bool,
+    suppress_vblank_this_frame: bool,
+    ppu_dot: u32,
+    odd_frame: bool,
+    in_vblank: bool,
+    frame_complete: bool,
+    mmc3_a12_prev_high: bool,
+    mmc3_a12_low_dots: u16,
+    mmc3_last_irq_scanline: u32,
+    mapper: Box<dyn Mapper + Send>,
+}
+
+impl NativeBus {
+    pub fn new(mapper: Box<dyn Mapper + Send>) -> Self {
+        Self {
+            cpu_ram: [0; CPU_RAM_SIZE],
+            ppu: Ppu::new(),
+            apu: Apu::new(),
+            cpu_open_bus: 0,
+            joypad_state: 0,
+            joypad_shift: 0,
+            joypad2_state: 0,
+            joypad2_shift: 0,
+            joypad_strobe: false,
+            nmi_pending: false,
+            suppress_vblank_this_frame: false,
+            ppu_dot: 0,
+            odd_frame: false,
+            in_vblank: false,
+            frame_complete: false,
+            mmc3_a12_prev_high: false,
+            mmc3_a12_low_dots: 8,
+            mmc3_last_irq_scanline: u32::MAX,
+            mapper,
+        }
+    }
+
+    pub fn apu_registers(&self) -> &[u8; 0x20] {
+        self.apu.registers()
+    }
+
+    pub fn render_frame(&self, out_rgba: &mut [u8], frame_number: u32, buttons: [bool; 8]) {
+        let _ = (frame_number, buttons);
+        let src = self.ppu.frame_buffer();
+        if out_rgba.len() >= src.len() {
+            out_rgba[..src.len()].copy_from_slice(src);
+        }
+    }
+
+    pub fn begin_frame(&mut self) {
+        self.frame_complete = false;
+        self.ppu.begin_frame();
+    }
+
+    pub fn take_frame_complete(&mut self) -> bool {
+        let out = self.frame_complete;
+        self.frame_complete = false;
+        out
+    }
+
+    pub fn clock_cpu(&mut self, cycles: u8) {
+        self.clock_cpu_cycles(cycles as u32);
+    }
+}
+
+// CpuBus trait implementation (memory map + side effects).
+mod cpu_bus_impl;
+mod joypad;
+// Save-state serialization helpers.
+mod state;
+mod timing;
+
+#[cfg(test)]
+mod tests;