Refactor and enhance documentation across the Wolf3D project

- Updated library imports to use the correct package paths for consistency.
- Added detailed documentation comments to various classes and methods, improving code readability and maintainability.
- Refined the GameSelectScreen, SpriteGallery, and VgaGallery classes with clearer descriptions of their functionality.
- Enhanced the CliInput class to better explain the input handling process and its interaction with the engine.
- Improved the SixelRasterizer and Opl2Emulator classes with comprehensive comments on their operations and state management.
- Removed the deprecated wolf_3d.dart file and consolidated its functionality into wolf_3d_flutter.dart for a cleaner architecture.
- Updated the Wolf3dFlutterInput class to clarify its role in merging keyboard and pointer events.
- Enhanced the rendering classes to provide better context on their purpose and usage within the Flutter framework.

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

apps/wolf_3d_gui/lib/screens/difficulty_screen.dart

@@ -1,11 +1,17 @@
+/// Difficulty picker shown after the player chooses an episode.
+library;
+
 import 'package:flutter/material.dart';
 import 'package:wolf_3d_dart/wolf_3d_data_types.dart';
-import 'package:wolf_3d_flutter/wolf_3d.dart';
+import 'package:wolf_3d_flutter/wolf_3d_flutter.dart';
 import 'package:wolf_3d_gui/screens/game_screen.dart';
 
+/// Starts a new game session using the active game and episode from [Wolf3d].
 class DifficultyScreen extends StatefulWidget {
+  /// Shared application facade carrying the active game, input, and audio.
   final Wolf3d wolf3d;
 
+  /// Creates the difficulty-selection screen for [wolf3d].
   const DifficultyScreen({
     super.key,
     required this.wolf3d,
@@ -25,6 +31,7 @@ class _DifficultyScreenState extends State<DifficultyScreen> {
     super.dispose();
   }
 
+  /// Replaces the menu flow with an active [GameScreen] using [difficulty].
   void _startGame(Difficulty difficulty, {bool showGallery = false}) {
     widget.wolf3d.audio.stopMusic();