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>

test/game/models/objective_test.dart

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+import 'package:block_seasons/game/engine/game_event.dart';
+import 'package:block_seasons/game/models/objective.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('ClearGems', () {
+    test('accumulates gems from clear events', () {
+      var obj = Objective.clearGems(6);
+      obj = obj.onEvent(const LinesCleared(lines: 1, gems: 2));
+      obj = obj.onEvent(const LinesCleared(lines: 2, gems: 3));
+      expect(obj.current, 5);
+      expect(obj.isComplete, isFalse);
+      obj = obj.onEvent(const LinesCleared(lines: 1, gems: 1));
+      expect(obj.isComplete, isTrue);
+    });
+
+    test('ignores unrelated events', () {
+      var obj = Objective.clearGems(3);
+      obj = obj.onEvent(const ScoreChanged(total: 9999));
+      expect(obj.current, 0);
+    });
+  });
+
+  group('ReachScore', () {
+    test('tracks the running total score', () {
+      var obj = Objective.reachScore(500);
+      obj = obj.onEvent(const ScoreChanged(total: 320));
+      expect(obj.current, 320);
+      expect(obj.isComplete, isFalse);
+      obj = obj.onEvent(const ScoreChanged(total: 510));
+      expect(obj.isComplete, isTrue);
+    });
+  });
+
+  group('ClearLines', () {
+    test('accumulates cleared lines', () {
+      var obj = Objective.clearLines(4);
+      obj = obj.onEvent(const LinesCleared(lines: 3, gems: 0));
+      expect(obj.current, 3);
+      obj = obj.onEvent(const LinesCleared(lines: 1, gems: 0));
+      expect(obj.isComplete, isTrue);
+    });
+  });
+
+  group('JSON', () {
+    test('round-trips all objective types', () {
+      for (final json in [
+        {'type': 'clearGems', 'count': 6},
+        {'type': 'reachScore', 'target': 1200},
+        {'type': 'clearLines', 'count': 10},
+      ]) {
+        final obj = Objective.fromJson(json);
+        expect(obj.toJson(), json);
+        expect(obj.current, 0);
+        expect(obj.target, greaterThan(0));
+      }
+    });
+
+    test('throws on unknown type', () {
+      expect(
+        () => Objective.fromJson({'type': 'collectStars', 'count': 1}),
+        throwsFormatException,
+      );
+    });
+  });
+}

test/game/models/stage_test.dart

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+import 'package:block_seasons/game/models/cell.dart';
+import 'package:block_seasons/game/models/objective.dart';
+import 'package:block_seasons/game/models/stage.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+const sampleJson = {
+  'id': 's001_007',
+  'seed': 8841273,
+  'moveLimit': 22,
+  'preset': [
+    {'x': 3, 'y': 4, 't': 'gem'},
+    {'x': 0, 'y': 7, 't': 'filled', 'c': 2},
+  ],
+  'objectives': [
+    {'type': 'clearGems', 'count': 6},
+  ],
+  'stars': {
+    'two': {'movesLeft': 4},
+    'three': {'movesLeft': 8},
+  },
+  'generatorProfile': 'mid',
+};
+
+void main() {
+  group('StageConfig', () {
+    test('parses the season pack stage schema', () {
+      final stage = StageConfig.fromJson(sampleJson);
+      expect(stage.id, 's001_007');
+      expect(stage.seed, 8841273);
+      expect(stage.moveLimit, 22);
+      expect(stage.preset, hasLength(2));
+      expect(stage.preset[0].type, CellType.gem);
+      expect(stage.preset[1].colorId, 2);
+      expect(stage.objectives.single, isA<Objective>());
+      expect(stage.generatorProfile, 'mid');
+    });
+
+    test('round-trips to JSON', () {
+      final stage = StageConfig.fromJson(sampleJson);
+      expect(stage.toJson(), sampleJson);
+    });
+
+    test('star thresholds map moves left to stars', () {
+      final stage = StageConfig.fromJson(sampleJson);
+      expect(stage.stars.starsFor(movesLeft: 0), 1);
+      expect(stage.stars.starsFor(movesLeft: 3), 1);
+      expect(stage.stars.starsFor(movesLeft: 4), 2);
+      expect(stage.stars.starsFor(movesLeft: 7), 2);
+      expect(stage.stars.starsFor(movesLeft: 8), 3);
+      expect(stage.stars.starsFor(movesLeft: 20), 3);
+    });
+
+    test('builds the initial grid from preset cells', () {
+      final stage = StageConfig.fromJson(sampleJson);
+      final grid = stage.initialGrid();
+      expect(grid.cellAt(3, 4).type, CellType.gem);
+      expect(grid.cellAt(0, 7).type, CellType.filled);
+      expect(grid.cellAt(0, 7).colorId, 2);
+      expect(grid.occupiedCount, 2);
+    });
+  });
+}