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/error.rs

@@ -0,0 +1,28 @@
+use crate::CpuError;
+use core::fmt;
+
+#[derive(Debug)]
+#[non_exhaustive]
+pub enum RuntimeError {
+    RomParse(String),
+    MapperInit(String),
+    Cpu(CpuError),
+    BufferTooSmall { expected: usize, got: usize },
+    InvalidState(String),
+}
+
+impl fmt::Display for RuntimeError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::RomParse(e) => write!(f, "ROM parse error: {e}"),
+            Self::MapperInit(e) => write!(f, "mapper init error: {e}"),
+            Self::Cpu(e) => write!(f, "CPU error: {e:?}"),
+            Self::BufferTooSmall { expected, got } => {
+                write!(f, "buffer too small: expected {expected} bytes, got {got}")
+            }
+            Self::InvalidState(e) => write!(f, "invalid runtime state: {e}"),
+        }
+    }
+}
+
+impl std::error::Error for RuntimeError {}