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/models/objective.dart

@@ -0,0 +1,88 @@
+import '../engine/game_event.dart';
+
+/// Stage win condition with immutable progress tracking. New objective types
+/// plug in as subclasses with a JSON tag.
+sealed class Objective {
+  const Objective();
+
+  factory Objective.clearGems(int count) => ClearGemsObjective(count, 0);
+  factory Objective.reachScore(int target) => ReachScoreObjective(target, 0);
+  factory Objective.clearLines(int count) => ClearLinesObjective(count, 0);
+
+  factory Objective.fromJson(Map<String, dynamic> json) {
+    switch (json['type']) {
+      case 'clearGems':
+        return Objective.clearGems(json['count'] as int);
+      case 'reachScore':
+        return Objective.reachScore(json['target'] as int);
+      case 'clearLines':
+        return Objective.clearLines(json['count'] as int);
+      default:
+        throw FormatException('Unknown objective type: ${json['type']}');
+    }
+  }
+
+  int get target;
+  int get current;
+  bool get isComplete => current >= target;
+
+  Objective onEvent(GameEvent event);
+
+  Map<String, dynamic> toJson();
+}
+
+class ClearGemsObjective extends Objective {
+  const ClearGemsObjective(this.target, this.current);
+
+  @override
+  final int target;
+  @override
+  final int current;
+
+  @override
+  Objective onEvent(GameEvent event) => switch (event) {
+        LinesCleared(:final gems) =>
+          ClearGemsObjective(target, current + gems),
+        _ => this,
+      };
+
+  @override
+  Map<String, dynamic> toJson() => {'type': 'clearGems', 'count': target};
+}
+
+class ReachScoreObjective extends Objective {
+  const ReachScoreObjective(this.target, this.current);
+
+  @override
+  final int target;
+  @override
+  final int current;
+
+  @override
+  Objective onEvent(GameEvent event) => switch (event) {
+        ScoreChanged(:final total) => ReachScoreObjective(target, total),
+        _ => this,
+      };
+
+  @override
+  Map<String, dynamic> toJson() => {'type': 'reachScore', 'target': target};
+}
+
+class ClearLinesObjective extends Objective {
+  const ClearLinesObjective(this.target, this.current);
+
+  @override
+  final int target;
+  @override
+  final int current;
+
+  @override
+  Objective onEvent(GameEvent event) => switch (event) {
+        LinesCleared(:final lines) =>
+          ClearLinesObjective(target, current + lines),
+        _ => this,
+      };
+
+  @override
+  Map<String, dynamic> toJson() => {'type': 'clearLines', 'count': target};
+}