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-api/src/lib.rs

@@ -0,0 +1,70 @@
+use std::collections::HashMap;
+
+pub const BUTTONS_COUNT: usize = 8;
+pub type ButtonState = [bool; BUTTONS_COUNT];
+
+pub trait InputSource {
+    fn poll_buttons(&mut self) -> ButtonState;
+}
+
+pub trait VideoSink {
+    fn present_rgba(&mut self, frame: &[u8], width: u32, height: u32);
+}
+
+pub trait AudioSink {
+    fn push_samples(&mut self, samples: &[f32]);
+}
+
+pub trait TimeSource {
+    fn wait_next_frame(&mut self);
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum StorageError {
+    NotFound(String),
+    Io(String),
+}
+
+pub trait FileStore {
+    fn read(&self, key: &str) -> Result<Vec<u8>, StorageError>;
+    fn write(&mut self, key: &str, bytes: &[u8]) -> Result<(), StorageError>;
+}
+
+#[derive(Default)]
+pub struct MemoryStore {
+    items: HashMap<String, Vec<u8>>,
+}
+
+impl FileStore for MemoryStore {
+    fn read(&self, key: &str) -> Result<Vec<u8>, StorageError> {
+        self.items
+            .get(key)
+            .cloned()
+            .ok_or_else(|| StorageError::NotFound(key.to_string()))
+    }
+
+    fn write(&mut self, key: &str, bytes: &[u8]) -> Result<(), StorageError> {
+        self.items.insert(key.to_string(), bytes.to_vec());
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{FileStore, MemoryStore, StorageError};
+
+    #[test]
+    fn memory_store_roundtrip() {
+        let mut store = MemoryStore::default();
+        store.write("slot1", &[1, 2, 3]).expect("write");
+        assert_eq!(store.read("slot1").expect("read"), vec![1, 2, 3]);
+    }
+
+    #[test]
+    fn memory_store_not_found() {
+        let store = MemoryStore::default();
+        let err = store.read("missing").expect_err("must fail");
+        assert!(matches!(err, StorageError::NotFound(_)));
+    }
+}