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/host/executor.rs

@@ -0,0 +1,63 @@
+use crate::runtime::{
+    AudioMixer, FRAME_HEIGHT, FRAME_RGBA_BYTES, FRAME_WIDTH, NesRuntime, RuntimeError,
+};
+
+use super::io::{AudioOutput, InputProvider, VideoOutput};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[non_exhaustive]
+pub struct FrameExecution {
+    pub frame_number: u64,
+    pub audio_samples: usize,
+}
+
+pub struct FrameExecutor {
+    mixer: AudioMixer,
+    frame_buffer: Vec<u8>,
+    audio_buffer: Vec<f32>,
+}
+
+impl FrameExecutor {
+    pub fn from_runtime(runtime: &NesRuntime, sample_rate: u32) -> Self {
+        Self {
+            mixer: runtime.default_audio_mixer(sample_rate),
+            frame_buffer: vec![0; FRAME_RGBA_BYTES],
+            audio_buffer: Vec::new(),
+        }
+    }
+
+    pub fn mixer(&self) -> &AudioMixer {
+        &self.mixer
+    }
+
+    pub fn mixer_mut(&mut self) -> &mut AudioMixer {
+        &mut self.mixer
+    }
+
+    pub fn execute_frame<I, V, A>(
+        &mut self,
+        runtime: &mut NesRuntime,
+        input: &mut I,
+        video: &mut V,
+        audio: &mut A,
+    ) -> Result<FrameExecution, RuntimeError>
+    where
+        I: InputProvider,
+        V: VideoOutput,
+        A: AudioOutput,
+    {
+        runtime.set_buttons(input.poll_buttons());
+        self.audio_buffer.clear();
+
+        runtime.run_until_frame_complete_with_audio(&mut self.mixer, &mut self.audio_buffer)?;
+        runtime.render_frame_rgba(&mut self.frame_buffer)?;
+
+        audio.push_samples(&self.audio_buffer);
+        video.present_rgba(&self.frame_buffer, FRAME_WIDTH, FRAME_HEIGHT);
+
+        Ok(FrameExecution {
+            frame_number: runtime.frame_number(),
+            audio_samples: self.audio_buffer.len(),
+        })
+    }
+}