fix(macos): prevent SIGILL crash on exit by replacing viewport close with clean process exit

### Summary
When selecting "Quit LightCraft" from the native macOS menu bar, the application occasionally crashes with an `EXC_BAD_INSTRUCTION` (SIGILL) error report during shutdown.

### Root Cause
1. The native menu action triggered `ctx.send_viewport_cmd(egui::ViewportCommand::Close)`.
2. Forcing an explicit viewport close via the command queue while macOS and AppKit are initiating their own termination sequence creates a race condition with `winit`.
3. This conflict during native window, view, and observer teardown leads to an illegal instruction fault.

### Solution
- Replaced the manual `ViewportCommand::Close` trigger on `QUIT` with a clean `std::process::exit(0)`.
- This bypasses the problematic `winit`/AppKit teardown race condition on macOS, allowing the application to shut down instantly and cleanly without generating crash reports.

apps/lightcraft/src/native_menu.rs

@@ -360,8 +360,7 @@ impl NativeMenu {
         lightcraft_ui_egui::i18n::set_language(app.ui.language);
         while let Ok(key) = self.rx.try_recv() {
             if key == QUIT {
-                ctx.send_viewport_cmd(egui::ViewportCommand::Close);
-                continue;
+                std::process::exit(0);
             }
             if key == "app.about" || key == SETTINGS {
                 let _ = app.run(&key, serde_json::json!({}));