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/constants.rs

@@ -0,0 +1,6 @@
+pub const FRAME_WIDTH: usize = 256;
+pub const FRAME_HEIGHT: usize = 240;
+pub const FRAME_RGBA_BYTES: usize = FRAME_WIDTH * FRAME_HEIGHT * 4;
+pub const SAVE_STATE_VERSION: u32 = 1;
+
+pub(crate) const SAVE_STATE_MAGIC: &[u8; 8] = b"NESRT001";