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.

.github/workflows/ci.yml

@@ -0,0 +1,35 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  rust:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install stable toolchain
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt, clippy
+
+      - name: Build
+        run: cargo build --locked
+
+      - name: Format
+        run: cargo fmt --all -- --check
+
+      - name: Clippy (strict)
+        run: cargo clippy-strict
+
+      - name: Test
+        run: cargo test --all-targets --all-features
+
+      - name: Public API Contract Tests
+        run: cargo test --test public_api --all-features
+
+      - name: Minimal Client Contract Tests
+        run: cargo test -p nesemu-client-minimal --test cli_contract