Add playable core UI: board painter, drag-and-drop, HUD, result overlay

CustomPainter board with gems/ghost/clear-flash, finger-lifted drag
with snap preview, combo text effect, HUD chips, phase overlays with
rescue stubs, demo stage. E2E widget test drives a real drag gesture.

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

lib/state/providers.dart

@@ -0,0 +1,8 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'game_session_notifier.dart';
+
+final gameSessionProvider =
+    NotifierProvider<GameSessionNotifier, GameViewState?>(
+  GameSessionNotifier.new,
+);