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/review_service.dart

@@ -0,0 +1,49 @@
+// lib/services/review_service.dart
+import '../data/save_repository.dart';
+import 'review_prompt_policy.dart';
+
+/// A thin seam over the in_app_review plugin so unit tests never reach platform
+/// channels. The release wiring uses [StoreReviewer] (see store_reviewer.dart),
+/// which forwards to the real InAppReview instance.
+abstract class Reviewer {
+  Future<bool> isAvailable();
+  Future<void> requestReview();
+}
+
+/// Decides and triggers the native "rate this app" sheet at most once, on a
+/// genuine high point. Holds no plugin dependency — that lives behind
+/// [Reviewer] — so the decision path is fully unit-testable.
+class ReviewService {
+  ReviewService({
+    required SaveRepository save,
+    required Reviewer reviewer,
+    ReviewPromptPolicy policy = const ReviewPromptPolicy(),
+  })  : _save = save,
+        _reviewer = reviewer,
+        _policy = policy;
+
+  final SaveRepository _save;
+  final Reviewer _reviewer;
+  final ReviewPromptPolicy _policy;
+
+  /// Call right after a stage win has been recorded to progress. Asks for a
+  /// review if the policy allows and the store can show the sheet, then never
+  /// again. The one-shot flag is only burned once the sheet is actually
+  /// requested, so an unavailable store retries on a later win. Every failure
+  /// is swallowed — a review prompt must never break gameplay.
+  Future<void> maybeRequestAfterWin({required int stars}) async {
+    final allowed = _policy.shouldRequest(
+      alreadyRequested: _save.reviewRequested,
+      won: true,
+      stars: stars,
+      totalStagesWon: _save.stagesClearedCount,
+    );
+    if (!allowed) return;
+    try {
+      if (await _reviewer.isAvailable()) {
+        await _reviewer.requestReview();
+        await _save.markReviewRequested();
+      }
+    } catch (_) {/* never break gameplay over a review prompt */}
+  }
+}