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/runtime/audio.rs

@@ -0,0 +1,39 @@
+use crate::runtime::VideoMode;
+
+#[derive(Debug)]
+pub struct AudioMixer {
+    sample_rate: u32,
+    samples_per_cpu_cycle: f64,
+    sample_accumulator: f64,
+}
+
+impl AudioMixer {
+    pub fn new(sample_rate: u32, mode: VideoMode) -> Self {
+        let cpu_hz = mode.cpu_hz();
+        Self {
+            sample_rate,
+            samples_per_cpu_cycle: sample_rate as f64 / cpu_hz,
+            sample_accumulator: 0.0,
+        }
+    }
+
+    pub fn sample_rate(&self) -> u32 {
+        self.sample_rate
+    }
+
+    pub fn reset(&mut self) {
+        self.sample_accumulator = 0.0;
+    }
+
+    pub fn push_cycles(&mut self, cpu_cycles: u8, apu_regs: &[u8; 0x20], out: &mut Vec<f32>) {
+        self.sample_accumulator += self.samples_per_cpu_cycle * f64::from(cpu_cycles);
+        let samples = self.sample_accumulator.floor() as usize;
+        self.sample_accumulator -= samples as f64;
+
+        // Current core does not expose a final mixed PCM stream yet.
+        // Use DMC output level as a stable interim signal in [-1.0, 1.0].
+        let dmc = apu_regs[0x11] & 0x7F;
+        let sample = (f32::from(dmc) / 63.5) - 1.0;
+        out.extend(std::iter::repeat_n(sample, samples));
+    }
+}