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/game/engine/game_engine.dart

@@ -20,12 +20,22 @@ class PlacementResult {
     required this.pointsGained,
     required this.linesCleared,
     required this.gemsCleared,
+    required this.clearedRows,
+    required this.clearedCols,
+    required this.comboStreak,
   });
 
   final List<GameEvent> events;
   final int pointsGained;
   final int linesCleared;
   final int gemsCleared;
+
+  /// Which lines vanished, for the UI clear animation.
+  final List<int> clearedRows;
+  final List<int> clearedCols;
+
+  /// Streak level after this placement, for combo effects.
+  final int comboStreak;
 }
 
 /// The single stateful pure-Dart session object for one stage attempt.
@@ -130,6 +140,9 @@ class GameEngine {
       pointsGained: delta.points,
       linesCleared: clear.linesCleared,
       gemsCleared: clear.gemsCleared,
+      clearedRows: clear.clearedRows,
+      clearedCols: clear.clearedCols,
+      comboStreak: _combo.streak,
     );
   }