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_prompt_policy.dart

@@ -0,0 +1,38 @@
+// lib/services/review_prompt_policy.dart
+
+/// Decides — as pure logic, with no plugin or storage dependency — whether the
+/// app should ask the player for a store review right now.
+///
+/// Store review prompts only land when they catch the player at a genuine high
+/// point, and both OSes hard-throttle how often the native sheet can appear.
+/// So we ask at most once (the caller persists [alreadyRequested]), and only
+/// after a clean, high-scoring win once the player is clearly invested.
+class ReviewPromptPolicy {
+  const ReviewPromptPolicy({
+    this.minStagesWon = 5,
+    this.requiredStars = 3,
+  });
+
+  /// How many stages the player must have cleared before we'll ask, so the
+  /// prompt never interrupts a newcomer still deciding if they like the game.
+  final int minStagesWon;
+
+  /// The star count the triggering win must reach — a top-marks finish is the
+  /// emotional peak we want to ride.
+  final int requiredStars;
+
+  /// True when a just-finished stage result should trigger the review prompt.
+  /// [alreadyRequested] is the persisted one-time guard owned by the caller.
+  bool shouldRequest({
+    required bool alreadyRequested,
+    required bool won,
+    required int stars,
+    required int totalStagesWon,
+  }) {
+    if (alreadyRequested) return false;
+    if (!won) return false;
+    if (stars < requiredStars) return false;
+    if (totalStagesWon < minStagesWon) return false;
+    return true;
+  }
+}