feat: analytics abstraction with debug backend and game event wiring

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

lib/services/analytics_service.dart

@@ -0,0 +1,64 @@
+import 'package:flutter/foundation.dart';
+
+/// Where events land. Phase 4 ships the debug logger; the Firebase backend
+/// plugs in here after the owner runs flutterfire configure.
+abstract class AnalyticsBackend {
+  void logEvent(String name, Map<String, Object> params);
+}
+
+class DebugAnalyticsBackend implements AnalyticsBackend {
+  @override
+  void logEvent(String name, Map<String, Object> params) {
+    debugPrint('[analytics] $name $params');
+  }
+}
+
+/// Typed event surface. Booleans are sent as 0/1 ints so every backend
+/// (GA4 included) aggregates them the same way.
+class AnalyticsService {
+  AnalyticsService(this._backend);
+
+  final AnalyticsBackend _backend;
+
+  void stageStart({required String seasonId, required String stageId}) {
+    _backend.logEvent('stage_start', {
+      'season_id': seasonId,
+      'stage_id': stageId,
+    });
+  }
+
+  void stageEnd({
+    required String seasonId,
+    required String stageId,
+    required bool won,
+    required int stars,
+    required int score,
+    required int movesUsed,
+  }) {
+    _backend.logEvent('stage_end', {
+      'season_id': seasonId,
+      'stage_id': stageId,
+      'won': won ? 1 : 0,
+      'stars': stars,
+      'score': score,
+      'moves_used': movesUsed,
+    });
+  }
+
+  void endlessStart() => _backend.logEvent('endless_start', const {});
+
+  void endlessEnd({required int score, required bool isNewBest}) {
+    _backend.logEvent('endless_end', {
+      'score': score,
+      'new_best': isNewBest ? 1 : 0,
+    });
+  }
+
+  void rescueUsed({required String type}) {
+    _backend.logEvent('rescue_used', {'type': type});
+  }
+
+  void tutorialFinished({required bool skipped}) {
+    _backend.logEvent('tutorial_finished', {'skipped': skipped ? 1 : 0});
+  }
+}