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/adapters.rs

@@ -0,0 +1,118 @@
+#[cfg(feature = "adapter-api")]
+use nesemu_adapter_api::{
+    AudioSink, BUTTONS_COUNT, ButtonState, InputSource, TimeSource, VideoSink,
+};
+
+#[cfg(feature = "adapter-api")]
+use crate::runtime::{FrameClock, JoypadButtons};
+
+#[cfg(feature = "adapter-api")]
+fn to_joypad_buttons(buttons: ButtonState) -> JoypadButtons {
+    let mut out = [false; crate::runtime::JOYPAD_BUTTONS_COUNT];
+    out.copy_from_slice(&buttons[..BUTTONS_COUNT]);
+    out
+}
+
+#[cfg(feature = "adapter-api")]
+pub struct InputAdapter<T> {
+    inner: T,
+}
+
+#[cfg(feature = "adapter-api")]
+impl<T> InputAdapter<T> {
+    pub const fn new(inner: T) -> Self {
+        Self { inner }
+    }
+
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+}
+
+#[cfg(feature = "adapter-api")]
+impl<T> crate::runtime::InputProvider for InputAdapter<T>
+where
+    T: InputSource,
+{
+    fn poll_buttons(&mut self) -> JoypadButtons {
+        to_joypad_buttons(self.inner.poll_buttons())
+    }
+}
+
+#[cfg(feature = "adapter-api")]
+pub struct VideoAdapter<T> {
+    inner: T,
+}
+
+#[cfg(feature = "adapter-api")]
+impl<T> VideoAdapter<T> {
+    pub const fn new(inner: T) -> Self {
+        Self { inner }
+    }
+
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+}
+
+#[cfg(feature = "adapter-api")]
+impl<T> crate::runtime::VideoOutput for VideoAdapter<T>
+where
+    T: VideoSink,
+{
+    fn present_rgba(&mut self, frame: &[u8], width: usize, height: usize) {
+        self.inner.present_rgba(frame, width as u32, height as u32);
+    }
+}
+
+#[cfg(feature = "adapter-api")]
+pub struct AudioAdapter<T> {
+    inner: T,
+}
+
+#[cfg(feature = "adapter-api")]
+impl<T> AudioAdapter<T> {
+    pub const fn new(inner: T) -> Self {
+        Self { inner }
+    }
+
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+}
+
+#[cfg(feature = "adapter-api")]
+impl<T> crate::runtime::AudioOutput for AudioAdapter<T>
+where
+    T: AudioSink,
+{
+    fn push_samples(&mut self, samples: &[f32]) {
+        self.inner.push_samples(samples);
+    }
+}
+
+#[cfg(feature = "adapter-api")]
+pub struct ClockAdapter<T> {
+    inner: T,
+}
+
+#[cfg(feature = "adapter-api")]
+impl<T> ClockAdapter<T> {
+    pub const fn new(inner: T) -> Self {
+        Self { inner }
+    }
+
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+}
+
+#[cfg(feature = "adapter-api")]
+impl<T> FrameClock for ClockAdapter<T>
+where
+    T: TimeSource,
+{
+    fn wait_next_frame(&mut self) {
+        self.inner.wait_next_frame();
+    }
+}