feat(review): request a store review after a 3-star win, once

Adds an in-app review prompt gated by ReviewPromptPolicy: only after a
3-star stage win, once the player has cleared >=5 stages, at most once
ever (persisted reviewRequested flag). ReviewService swallows all
failures and only burns the one-shot when the store actually shows the
sheet, so an unavailable store retries on a later win. StoreReviewer
wraps in_app_review behind a Reviewer seam so unit tests skip platform
channels. 13 new tests; full suite 194 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/services/store_reviewer.dart

@@ -0,0 +1,21 @@
+// lib/services/store_reviewer.dart
+import 'package:in_app_review/in_app_review.dart';
+
+import 'review_service.dart';
+
+/// Production [Reviewer] that forwards to the in_app_review plugin's native
+/// "rate this app" sheet (SKStoreReviewController on iOS, In-App Review API on
+/// Android). Kept in its own file so [ReviewService] and its tests stay free of
+/// the platform-channel dependency.
+class StoreReviewer implements Reviewer {
+  StoreReviewer([InAppReview? inAppReview])
+      : _inAppReview = inAppReview ?? InAppReview.instance;
+
+  final InAppReview _inAppReview;
+
+  @override
+  Future<bool> isAvailable() => _inAppReview.isAvailable();
+
+  @override
+  Future<void> requestReview() => _inAppReview.requestReview();
+}