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/native_core/mapper/mappers/fme7.rs

@@ -0,0 +1,19 @@
+use super::*;
+
+pub(crate) struct Fme7 {
+    pub(super) prg_rom: Vec<u8>,
+    pub(super) chr_data: Vec<u8>,
+    pub(super) chr_is_ram: bool,
+    pub(super) mirroring: Mirroring,
+    pub(super) command: u8,
+    pub(super) chr_banks: [u8; 8],
+    pub(super) prg_banks: [u8; 3],
+    pub(super) low_bank: u8,
+    pub(super) low_is_ram: bool,
+    pub(super) low_ram_enabled: bool,
+    pub(super) low_ram: Vec<u8>,
+    pub(super) irq_counter: u16,
+    pub(super) irq_enabled: bool,
+    pub(super) irq_counter_enabled: bool,
+    pub(super) irq_pending: bool,
+}