Add synthesized SFX and audio wiring

Pure-Dart WAV synthesizer (tool/gen_sfx.dart) generates place/clear/
combo/win/lose effects; AudioService player pool fires on placement,
line clears, combo streaks, and phase transitions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pubspec.yaml

@@ -63,6 +63,9 @@ flutter:
   # the material Icons class.
   uses-material-design: true
 
+  assets:
+    - assets/audio/
+
   # To add assets to your application, add an assets section, like this:
   # assets:
   #   - images/a_dot_burr.jpeg