Refactor menu structure and add Flutter-specific input and persistence layers

- Moved menu-related classes to a new structure under `src/menu/`.
- Introduced `WolfMenuPresentation` to handle menu art and mappings.
- Added `MenuManager` tests to ensure menu state reflects game status.
- Implemented `FlutterRendererSettingsPersistence` and `FlutterSaveGamePersistence` for managing settings and save files on desktop platforms.
- Created `Wolf3dFlutterInput` to handle keyboard and mouse input in a Flutter environment.
- Updated README to reflect new package structure and usage instructions.

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

packages/wolf_3d_dart/test/registry/spear_demo_registry_test.dart

@@ -1,4 +1,5 @@
 import 'package:test/test.dart';
+import 'package:wolf_3d_dart/src/registry/built_in/spear_menu_presentation_module.dart';
 import 'package:wolf_3d_dart/wolf_3d_data_types.dart';
 import 'package:wolf_3d_dart/wolf_3d_entities.dart';
 
@@ -17,6 +18,7 @@ void main() {
       expect(registry.hud.resolve(HudKey.statusBar)?.vgaIndex, 73);
       expect(registry.hud.resolve(HudKey.digit0)?.vgaIndex, 84);
       expect(registry.hud.resolve(HudKey.pistolIcon)?.vgaIndex, 77);
+      expect(registry.menuPresentation, isA<SpearMenuPresentationModule>());
     });
 
     test('resolves SDM enemy sprite ranges with +4 SPEAR shift', () {

packages/wolf_3d_dart/test/registry/wolf_classic_menu_art_mapping_test.dart

@@ -21,14 +21,14 @@ void main() {
         vgaImages: List.generate(120, (i) => _img(height: i + 1)),
       );
 
-      final art = WolfClassicMenuArt(data);
+      final menu = WolfMenuPresentation(data);
 
       // Retail cursor constants 8/9 must resolve to shareware cursor IDs 20/21.
-      expect(art.mappedPic(8)?.height, 21);
-      expect(art.mappedPic(9)?.height, 22);
+      expect(menu.mappedPic(8)?.height, 21);
+      expect(menu.mappedPic(9)?.height, 22);
 
       // Retail footer constant 15 must resolve to shareware footer ID 27.
-      expect(art.mappedPic(15)?.height, 28);
+      expect(menu.mappedPic(15)?.height, 28);
     },
   );
 }