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.

crates/nesemu-adapter-headless/src/lib.rs

@@ -0,0 +1,52 @@
+use nesemu_adapter_api::{
+    AudioSink, BUTTONS_COUNT, ButtonState, InputSource, TimeSource, VideoSink,
+};
+
+#[derive(Default)]
+pub struct NullInput;
+
+impl InputSource for NullInput {
+    fn poll_buttons(&mut self) -> ButtonState {
+        [false; BUTTONS_COUNT]
+    }
+}
+
+#[derive(Default)]
+pub struct NullVideo;
+
+impl VideoSink for NullVideo {
+    fn present_rgba(&mut self, _frame: &[u8], _width: u32, _height: u32) {}
+}
+
+#[derive(Default)]
+pub struct NullAudio;
+
+impl AudioSink for NullAudio {
+    fn push_samples(&mut self, _samples: &[f32]) {}
+}
+
+#[derive(Default)]
+pub struct NoopTime;
+
+impl TimeSource for NoopTime {
+    fn wait_next_frame(&mut self) {}
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{NoopTime, NullAudio, NullInput, NullVideo};
+    use nesemu_adapter_api::{AudioSink, BUTTONS_COUNT, InputSource, TimeSource, VideoSink};
+
+    #[test]
+    fn null_adapters_are_noop() {
+        let mut input = NullInput;
+        let mut video = NullVideo;
+        let mut audio = NullAudio;
+        let mut time = NoopTime;
+
+        assert_eq!(input.poll_buttons(), [false; BUTTONS_COUNT]);
+        video.present_rgba(&[], 256, 240);
+        audio.push_samples(&[]);
+        time.wait_next_frame();
+    }
+}