Add objectives, stage config, and GameEngine session core

Sealed Objective types (clearGems/reachScore/clearLines) with JSON
round-trip; StageConfig with preset cells and star thresholds;
GameEngine orchestrating placement -> clear -> scoring -> objectives
with stuck detection, one-shot rescue (continue / +5 moves), and
deterministic per-attempt RNG. 100-game headless stress test and
pure-Dart architecture guard. 76 tests green.

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

lib/game/engine/game_event.dart

@@ -0,0 +1,28 @@
+import '../models/piece.dart';
+
+/// Events emitted by the engine during a placement, consumed by objectives
+/// and (later) the UI effects layer.
+sealed class GameEvent {
+  const GameEvent();
+}
+
+class PiecePlaced extends GameEvent {
+  const PiecePlaced({required this.piece, required this.x, required this.y});
+
+  final Piece piece;
+  final int x;
+  final int y;
+}
+
+class LinesCleared extends GameEvent {
+  const LinesCleared({required this.lines, required this.gems});
+
+  final int lines;
+  final int gems;
+}
+
+class ScoreChanged extends GameEvent {
+  const ScoreChanged({required this.total});
+
+  final int total;
+}