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/architecture_test.dart

@@ -0,0 +1,24 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+
+/// The game engine must stay pure Dart: unit-testable headless, reusable by
+/// the stage generator CLI, and free of widget-layer coupling.
+void main() {
+  test('lib/game and lib/core never import Flutter', () {
+    final dirs = [Directory('lib/game'), Directory('lib/core')];
+    final violations = <String>[];
+    for (final dir in dirs) {
+      for (final entity in dir.listSync(recursive: true)) {
+        if (entity is! File || !entity.path.endsWith('.dart')) continue;
+        final source = entity.readAsStringSync();
+        if (source.contains("import 'package:flutter") ||
+            source.contains('import "package:flutter')) {
+          violations.add(entity.path);
+        }
+      }
+    }
+    expect(violations, isEmpty,
+        reason: 'Flutter imports found in pure-Dart layers: $violations');
+  });
+}