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/timing.rs

@@ -0,0 +1,116 @@
+use super::{
+    NativeBus, PPU_DOTS_PER_FRAME, PPU_DOTS_PER_SCANLINE, PPU_PRERENDER_SCANLINE,
+    PPU_VBLANK_START_SCANLINE,
+};
+use crate::native_core::cpu::CpuBus;
+use crate::native_core::mapper::Mapper;
+
+impl NativeBus {
+    fn clock_one_cpu_cycle(&mut self) {
+        for _ in 0..3 {
+            self.clock_ppu_dot();
+        }
+        self.mapper.clock_cpu(1);
+        self.apu.clock_cpu_cycle();
+    }
+
+    fn service_dmc_dma(&mut self, addr: u16) {
+        let byte = self.dma_read(addr);
+        self.apu.provide_dmc_dma_byte(byte);
+        // DMC DMA steals CPU bus cycles while APU/PPU/mapper keep ticking.
+        for _ in 0..4 {
+            self.clock_one_cpu_cycle();
+        }
+    }
+
+    pub(super) fn dma_read(&mut self, addr: u16) -> u8 {
+        <Self as CpuBus>::read(self, addr)
+    }
+
+    pub(super) fn clock_ppu_dot(&mut self) {
+        self.ppu_dot += 1;
+        if self.ppu_dot >= PPU_DOTS_PER_FRAME {
+            self.ppu_dot = 0;
+            self.frame_complete = true;
+            self.odd_frame = !self.odd_frame;
+        }
+
+        let scanline = self.ppu_dot / PPU_DOTS_PER_SCANLINE;
+        let dot = self.ppu_dot % PPU_DOTS_PER_SCANLINE;
+        let rendering_enabled = self.ppu.rendering_enabled();
+
+        {
+            let mapper: &(dyn Mapper + Send) = &*self.mapper;
+            self.ppu.render_dot(mapper, scanline, dot);
+        }
+
+        if rendering_enabled && (scanline < 240 || scanline == PPU_PRERENDER_SCANLINE) {
+            if self.mapper.needs_ppu_a12_clock() {
+                let a12_high = self.ppu.mmc3_a12_high_at(scanline, dot);
+                if a12_high {
+                    if !self.mmc3_a12_prev_high
+                        && self.mmc3_a12_low_dots >= 8
+                        && self.mmc3_last_irq_scanline != scanline
+                    {
+                        self.mapper.clock_scanline();
+                        self.mmc3_last_irq_scanline = scanline;
+                    }
+                    self.mmc3_a12_prev_high = true;
+                    self.mmc3_a12_low_dots = 0;
+                } else {
+                    self.mmc3_a12_prev_high = false;
+                    self.mmc3_a12_low_dots = self.mmc3_a12_low_dots.saturating_add(1);
+                }
+            } else if dot == 260 {
+                self.mapper.clock_scanline();
+            }
+        } else {
+            self.mmc3_a12_prev_high = false;
+            self.mmc3_a12_low_dots = self.mmc3_a12_low_dots.saturating_add(1);
+        }
+        if rendering_enabled && scanline == PPU_PRERENDER_SCANLINE && dot == 339 && self.odd_frame {
+            // NTSC odd frame timing: skip pre-render dot 340 when rendering is enabled.
+            self.ppu_dot = self.ppu_dot.saturating_add(1);
+        }
+
+        if !self.in_vblank && scanline == PPU_VBLANK_START_SCANLINE && dot == 1 {
+            if self.suppress_vblank_this_frame {
+                self.suppress_vblank_this_frame = false;
+                self.in_vblank = false;
+                self.ppu.set_vblank(false);
+            } else {
+                self.in_vblank = true;
+                self.ppu.set_vblank(true);
+                if self.ppu.nmi_enabled() {
+                    self.nmi_pending = true;
+                }
+            }
+        } else if scanline == PPU_PRERENDER_SCANLINE && dot == 1 {
+            self.in_vblank = false;
+            self.ppu.set_vblank(false);
+            self.ppu.set_sprite0_hit(false);
+            self.ppu.set_sprite_overflow(false);
+            self.suppress_vblank_this_frame = false;
+        }
+    }
+
+    pub(super) fn clock_cpu_cycles(&mut self, cycles: u32) {
+        if cycles == 0 {
+            return;
+        }
+        let mut remaining = cycles;
+        while remaining != 0 {
+            self.clock_one_cpu_cycle();
+            while let Some(addr) = self.apu.take_dmc_dma_request() {
+                self.service_dmc_dma(addr);
+            }
+            remaining -= 1;
+        }
+    }
+
+    pub(super) fn note_scroll_write_now(&mut self) {
+        let scanline = (self.ppu_dot / PPU_DOTS_PER_SCANLINE) as usize;
+        let dot = self.ppu_dot % PPU_DOTS_PER_SCANLINE;
+        self.ppu.note_scroll_register_write(scanline, dot);
+    }
+}